boiler safety valve diagram in stock

There is a wide range of safety valves available to meet the many different applications and performance criteria demanded by different industries. Furthermore, national standards define many varying types of safety valve.

The ASME standard I and ASME standard VIII for boiler and pressure vessel applications and the ASME/ANSI PTC 25.3 standard for safety valves and relief valves provide the following definition. These standards set performance characteristics as well as defining the different types of safety valves that are used:

ASME I valve - A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will open within 3% overpressure and close within 4%. It will usually feature two blowdown rings, and is identified by a National Board ‘V’ stamp.

ASME VIII valve- A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will open within 10% overpressure and close within 7%. Identified by a National Board ‘UV’ stamp.

Full bore safety valve - A safety valve having no protrusions in the bore, and wherein the valve lifts to an extent sufficient for the minimum area at any section, at or below the seat, to become the controlling orifice.

Conventional safety relief valve -The spring housing is vented to the discharge side, hence operational characteristics are directly affected by changes in the backpressure to the valve.

Balanced safety relief valve -A balanced valve incorporates a means of minimising the effect of backpressure on the operational characteristics of the valve.

Pilot operated pressure relief valve -The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device.

Power-actuated safety relief valve - A pressure relief valve in which the major pressure relieving device is combined with, and controlled by, a device requiring an external source of energy.

Standard safety valve - A valve which, following opening, reaches the degree of lift necessary for the mass flowrate to be discharged within a pressure rise of not more than 10%. (The valve is characterised by a pop type action and is sometimes known as high lift).

Full lift (Vollhub) safety valve -A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%.

Direct loaded safety valve -A safety valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or a weight.

Proportional safety valve - A safety valve which opens more or less steadily in relation to the increase in pressure. Sudden opening within a 10% lift range will not occur without pressure increase. Following opening within a pressure of not more than 10%, these safety valves achieve the lift necessary for the mass flow to be discharged.

Diaphragm safety valve -A direct loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm

Bellows safety valve - A direct loaded safety valve wherein sliding and (partially or fully) rotating elements and springs are protected against the effects of the fluids by a bellows. The bellows may be of such a design that it compensates for influences of backpressure.

Controlled safety valve - Consists of a main valve and a control device. It also includes direct acting safety valves with supplementary loading in which, until the set pressure is reached, an additional force increases the closing force.

Safety valve - A safety valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored. Note; the valve can be characterised either by pop action (rapid opening) or by opening in proportion (not necessarily linear) to the increase in pressure over the set pressure.

Direct loaded safety valve -A safety valve in which the loading due to the fluid pressure underneath the valve disc is opposed only by a direct mechanical loading device such as a weight, lever and weight, or a spring.

Assisted safety valve -A safety valve which by means of a powered assistance mechanism, may additionally be lifted at a pressure lower than the set pressure and will, even in the event of a failure of the assistance mechanism, comply with all the requirements for safety valves given in the standard.

Supplementary loaded safety valve - A safety valve that has, until the pressure at the inlet to the safety valve reaches the set pressure, an additional force, which increases the sealing force.

Note; this additional force (supplementary load), which may be provided by means of an extraneous power source, is reliably released when the pressure at the inlet of the safety valve reaches the set pressure. The amount of supplementary loading is so arranged that if such supplementary loading is not released, the safety valve will attain its certified discharge capacity at a pressure not greater than 1.1 times the maximum allowable pressure of the equipment to be protected.

Pilot operated safety valve -A safety valve, the operation of which is initiated and controlled by the fluid discharged from a pilot valve, which is itself, a direct loaded safety valve subject to the requirement of the standard.

The common characteristic shared between the definitions of conventional safety valves in the different standards, is that their operational characteristics are affected by any backpressure in the discharge system. It is important to note that the total backpressure is generated from two components; superimposed backpressure and the built-up backpressure:

Subsequently, in a conventional safety valve, only the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the blowdown characteristic and re-seat value.

The ASME/ANSI standard makes the further classification that conventional valves have a spring housing that is vented to the discharge side of the valve. If the spring housing is vented to the atmosphere, any superimposed backpressure will still affect the operational characteristics. Thiscan be seen from Figure 9.2.1, which shows schematic diagrams of valves whose spring housings are vented to the discharge side of the valve and to the atmosphere.

By considering the forces acting on the disc (with area AD), it can be seen that the required opening force (equivalent to the product of inlet pressure (PV) and the nozzle area (AN)) is the sum of the spring force (FS) and the force due to the backpressure (PB) acting on the top and bottom of the disc. In the case of a spring housing vented to the discharge side of the valve (an ASME conventional safety relief valve, see Figure 9.2.1 (a)), the required opening force is:

In both cases, if a significant superimposed backpressure exists, its effects on the set pressure need to be considered when designing a safety valve system.

Once the valve starts to open, the effects of built-up backpressure also have to be taken into account. For a conventional safety valve with the spring housing vented to the discharge side of the valve, see Figure 9.2.1 (a), the effect of built-up backpressure can be determined by considering Equation 9.2.1 and by noting that once the valve starts to open, the inlet pressure is the sum of the set pressure, PS, and the overpressure, PO.

In both cases, if a significant superimposed backpressure exists, its effects on the set pressure need to be considered when designing a safety valve system.

Once the valve starts to open, the effects of built-up backpressure also have to be taken into account. For a conventional safety valve with the spring housing vented to the discharge side of the valve, see Figure 9.2.1 (a), the effect of built-up backpressure can be determined by considering Equation 9.2.1 and by noting that once the valve starts to open, the inlet pressure is the sum of the set pressure, PS, and the overpressure, PO.

Balanced safety valves are those that incorporate a means of eliminating the effects of backpressure. There are two basic designs that can be used to achieve this:

Although there are several variations of the piston valve, they generally consist of a piston type disc whose movement is constrained by a vented guide. The area of the top face of the piston, AP, and the nozzle seat area, AN, are designed to be equal. This means that the effective area of both the top and bottom surfaces of the disc exposed to the backpressure are equal, and therefore any additional forces are balanced. In addition, the spring bonnet is vented such that the top face of the piston is subjected to atmospheric pressure, as shown in Figure 9.2.2.

The bellows arrangement prevents backpressure acting on the upper side of the disc within the area of the bellows. The disc area extending beyond the bellows and the opposing disc area are equal, and so the forces acting on the disc are balanced, and the backpressure has little effect on the valve opening pressure.

Bellows failure is an important concern when using a bellows balanced safety valve, as this may affect the set pressure and capacity of the valve. It is important, therefore, that there is some mechanism for detecting any uncharacteristic fluid flow through the bellows vents. In addition, some bellows balanced safety valves include an auxiliary piston that is used to overcome the effects of backpressure in the case of bellows failure. This type of safety valve is usually only used on critical applications in the oil and petrochemical industries.

Since balanced pressure relief valves are typically more expensive than their unbalanced counterparts, they are commonly only used where high pressure manifolds are unavoidable, or in critical applications where a very precise set pressure or blowdown is required.

This type of safety valve uses the flowing medium itself, through a pilot valve, to apply the closing force on the safety valve disc. The pilot valve is itself a small safety valve.

The diaphragm type is typically only available for low pressure applications and it produces a proportional type action, characteristic of relief valves used in liquid systems. They are therefore of little use in steam systems, consequently, they will not be considered in this text.

The piston type valve consists of a main valve, which uses a piston shaped closing device (or obturator), and an external pilot valve. Figure 9.2.4 shows a diagram of a typical piston type, pilot operated safety valve.

The piston and seating arrangement incorporated in the main valve is designed so that the bottom area of the piston, exposed to the inlet fluid, is less than the area of the top of the piston. As both ends of the piston are exposed to the fluid at the same pressure, this means that under normal system operating conditions, the closing force, resulting from the larger top area, is greater than the inlet force. The resultant downward force therefore holds the piston firmly on its seat.

If the inlet pressure were to rise, the net closing force on the piston also increases, ensuring that a tight shut-off is continually maintained. However, when the inlet pressure reaches the set pressure, the pilot valve will pop open to release the fluid pressure above the piston. With much less fluid pressure acting on the upper surface of the piston, the inlet pressure generates a net upwards force and the piston will leave its seat. This causes the main valve to pop open, allowing the process fluid to be discharged.

When the inlet pressure has been sufficiently reduced, the pilot valve will reclose, preventing the further release of fluid from the top of the piston, thereby re-establishing the net downward force, and causing the piston to reseat.

Pilot operated safety valves offer good overpressure and blowdown performance (a blowdown of 2% is attainable). For this reason, they are used where a narrow margin is required between the set pressure and the system operating pressure. Pilot operated valves are also available in much larger sizes, making them the preferred type of safety valve for larger capacities.

One of the main concerns with pilot operated safety valves is that the small bore, pilot connecting pipes are susceptible to blockage by foreign matter, or due to the collection of condensate in these pipes. This can lead to the failure of the valve, either in the open or closed position, depending on where the blockage occurs.

The terms full lift, high lift and low lift refer to the amount of travel the disc undergoes as it moves from its closed position to the position required to produce the certified discharge capacity, and how this affects the discharge capacity of the valve.

A full lift safety valve is one in which the disc lifts sufficiently, so that the curtain area no longer influences the discharge area. The discharge area, and therefore the capacity of the valve are subsequently determined by the bore area. This occurs when the disc lifts a distance of at least a quarter of the bore diameter. A full lift conventional safety valve is often the best choice for general steam applications.

The disc of a high lift safety valve lifts a distance of at least 1/12th of the bore diameter. This means that the curtain area, and ultimately the position of the disc, determines the discharge area. The discharge capacities of high lift valves tend to be significantly lower than those of full lift valves, and for a given discharge capacity, it is usually possible to select a full lift valve that has a nominal size several times smaller than a corresponding high lift valve, which usually incurs cost advantages.Furthermore, high lift valves tend to be used on compressible fluids where their action is more proportional.

In low lift valves, the disc only lifts a distance of 1/24th of the bore diameter. The discharge area is determined entirely by the position of the disc, and since the disc only lifts a small amount, the capacities tend to be much lower than those of full or high lift valves.

Except when safety valves are discharging, the only parts that are wetted by the process fluid are the inlet tract (nozzle) and the disc. Since safety valves operate infrequently under normal conditions, all other components can be manufactured from standard materials for most applications. There are however several exceptions, in which case, special materials have to be used, these include:

Cast steel -Commonly used on higher pressure valves (up to 40 bar g). Process type valves are usually made from a cast steel body with an austenitic full nozzle type construction.

For all safety valves, it is important that moving parts, particularly the spindle and guides are made from materials that will not easily degrade or corrode. As seats and discs are constantly in contact with the process fluid, they must be able to resist the effects of erosion and corrosion.

The spring is a critical element of the safety valve and must provide reliable performance within the required parameters. Standard safety valves will typically use carbon steel for moderate temperatures. Tungsten steel is used for higher temperature, non-corrosive applications, and stainless steel is used for corrosive or clean steam duty. For sour gas and high temperature applications, often special materials such as monel, hastelloy and ‘inconel’ are used.

Standard safety valves are generally fitted with an easing lever, which enables the valve to be lifted manually in order to ensure that it is operational at pressures in excess of 75% of set pressure. This is usually done as part of routine safety checks, or during maintenance to prevent seizing. The fitting of a lever is usually a requirement of national standards and insurance companies for steam and hot water applications. For example, the ASME Boiler and Pressure Vessel Code states that pressure relief valves must be fitted with a lever if they are to be used on air, water over 60°C, and steam.

A test gag (Figure 9.2.7) may be used to prevent the valve from opening at the set pressure during hydraulic testing when commissioning a system. Once tested, the gag screw is removed and replaced with a short blanking plug before the valve is placed in service.

The amount of fluid depends on the particular design of safety valve. If emission of this fluid into the atmosphere is acceptable, the spring housing may be vented to the atmosphere – an open bonnet. This is usually advantageous when the safety valve is used on high temperature fluids or for boiler applications as, otherwise, high temperatures can relax the spring, altering the set pressure of the valve. However, using an open bonnet exposes the valve spring and internals to environmental conditions, which can lead to damage and corrosion of the spring.

When the fluid must be completely contained by the safety valve (and the discharge system), it is necessary to use a closed bonnet, which is not vented to the atmosphere. This type of spring enclosure is almost universally used for small screwed valves and, it is becoming increasingly common on many valve ranges since, particularly on steam, discharge of the fluid could be hazardous to personnel.

Some safety valves, most commonly those used for water applications, incorporate a flexible diaphragm or bellows to isolate the safety valve spring and upper chamber from the process fluid, (see Figure 9.2.9).

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Above certification pressure no one can guaranty the systems safety - and especially for a steam system with a very hot gas with a huge amount of latent heat the consequence with a failure can be dramatically.

The size of a safety valve depends primarily on the maximum boiler output and the operation pressure of the system. The safety valve must as minimum have the evacuation capacity of all the vapor the boiler can produce running at full power at the working (or certification) pressure. for a higher pressure the steam is compressed and requires less volume and the size of the valve can be reduced

The tables below can be used to select a typical safety valve in a high pressure system. Before the final design - always consult manufacturing documentation.

The main purpose of a safety valve is to prevent the pressure in a system to exceed the certification pressure. Above certification pressure, no one can guaranty the systems safety - and especially for a steam system with very hot gas with huge amount of latent heat, the consequences can be dramatically.

The size of the safety valve depends primarily on the maximum boiler output and the operation pressure of the system. The safety valve shall as minimum have the evacuation capacity of all the vapor the boiler can produce running at full power at working (or certification) pressure.

The table below can be used to select a typical safety valve based on boiler output. Before final design, always consult the manufactures documentation.

Note! The table above is based on low pressure steam of 100 kN/m2 (1 bar)or 15 psiin imperial units. Latent heat of saturated steam is 2201 kJ/kg (945 Btu/lb). 1 N/m2 = 1 Pa = 1.4504 x 10-4 lb/in2 (psi) = 10-5 bar For higher pressure, steam is compressed and require less volume - required size of the valve reduced

Years ago, it was not uncommon to read news about tragic boiler explosions, sometimes resulting in mass destruction. Today, boilers are equipped with important safety devises to help protect against these types of catastrophes. Let’s take a look at the most critical of these devices: the safety valve.

The safety valve is one of the most important safety devices in a steam system. Safety valves provide a measure of security for plant operators and equipment from over pressure conditions. The main function of a safety valve is to relieve pressure. It is located on the boiler steam drum, and will automatically open when the pressure of the inlet side of the valve increases past the preset pressure. All boilers are required by ASME code to have at least one safety valve, dependent upon the maximum flow capacity (MFC) of the boiler. The total capacity of the safety valve at the set point must exceed the steam control valve’s MFC if the steam valve were to fail to open. In most cases, two safety valves per boiler are required, and a third may be needed if they do not exceed the MFC.

There are three main parts to the safety valve: nozzle, disc, and spring. Pressurized steam enters the valve through the nozzle and is then threaded to the boiler. The disc is the lid to the nozzle, which opens or closes depending on the pressure coming from the boiler. The spring is the pressure controller.

As a boiler starts to over pressure, the nozzle will start to receive a higher pressure coming from the inlet side of the valve, and will start to sound like it is simmering. When the pressure becomes higher than the predetermined pressure of the spring, the disc will start to lift and release the steam, creating a “pop” sound. After it has released and the steam and pressure drops below the set pressure of the valve, the spring will close the disc. Once the safety valve has popped, it is important to check the valve to make sure it is not damaged and is working properly.

A safety valve is usually referred to as the last line of safety defense. Without safety valves, the boiler can exceed it’s maximum allowable working pressure (MAWP) and not only damage equipment, but also injure or kill plant operators that are close by. Many variables can cause a safety valve on a boiler to lift, such as a compressed air or electrical power failure to control instrumentation, or an imbalance of feedwater rate caused by an inadvertently shut or open isolation valve.

Once a safety valve has lifted, it is important to do a complete boiler inspection and confirm that there are no other boiler servicing issues. A safety valve should only do its job once; safety valves should not lift continuously. Lastly, it is important to have the safety valves fully repaired, cleaned and recertified with a National Board valve repair (VR) stamp as required by local code or jurisdiction. Safety valves are a critical component in a steam system, and must be maintained.

All of Nationwide Boiler’s rental boilers include on to two safety valves depending on the size; one set at design pressure and the other set slightly higher than design. By request, we can reset the safeties to a lower pressure if the application requires it. In addition, the valves are thoroughly checked after every rental and before going out to a new customer, and they are replaced and re-certified as needed.

When carrying out water leak detection work something which frequently comes into consideration is understanding Pressure Relief Valves (PRVs) on boilers, Which are not to be confused with pressure reducing valves (also called PRVs in the plumbing world!), which we will explain about later.

Pressure relief valves especially come into the equation when someone has a boiler pressure dropping causing it to turn off and central heating not to work. In such situations people sometimes find themselves needing to top up their boiler pressure in order to get it working again.

Although this it is not an especially uncommon occurrence, doing it regularly suggests a problem with the central heating system, which could be associated with a water leak. As we have discussed before this can be caused by a number of factors one of which is possibly a problem with the pressure relief valve.

In some respects a pressure release valve explains itself what it does but let’s help with a simple explanation, other than ‘a device to relieve pressure’.

Pressure relief valves are effectively a safety mechanism for the boiler and central heating system To relieve pressure on the system when it gets too high. as you can see from the image below, pressure relief valves are set at a specific pressure level, often 3 bar, like this one below:

It is quickly worth mentioning as usual that exactly how boilers and central heating systems are set up will vary according to the type of boiler and how is being installed.

That said pressurised or sealed central heating systems should have a pressure relief valve. as we explained in our article about boilers losing pressure, pressurised boiler systems have an optimal range of pressure they operate in. The actual operating pressure of the boiler will vary from time to time including when it is on versus when it is off on many occasions

Basically, if the pressure on the system reaches the calibrated level it releases some water out of the system to reduce the pressure then sealing again once the pressure has reduced to normal levels. Notice how, given their importance as a safety feature that the have a CE Marking (CE1115).

With Pressure Relief Valves, some people call them pressure release valves, some even call them PRV valves (the ‘V’ obviously stands for valve already) but effectively they are talking about the same thing usually when it comes to boilers leaking a subset of which is combi boilers leaking if a pressure relief valve leaking on a combi boiler in particular.

Just to confuse matters more, some people refer to them as Safety Relief Valves (SRVs)or even Safety Release Valves! Remember also that these terms are used for applications other than central heating boilers, for example on an air compressor or even industrial machinery. More on that later.

To help explain how a pressure relief valve works we have put together this basic diagram above showing the main component parts and how it works. This includes:

As we said previously, each pressure relief valve is calibrated to a specific pressure and the key component of this is the calibrated spring within the body of the PRV.

As a result of this, water will be being released from the pressurised system dropping the boiler system pressure which over time may cause it to stop working and require being repressurised.

In many situations the pressure relief valve itself is attached in and around the pipework close to the boiler, and although there are many different designs, commonly they look like the pictures shown in this article. They are usually accessible so that boiler engineers etc can’t access them to inspect, repair or replace them.

If a pressure relief valve has been triggered and released water that can in some instances be a trigger for debris to get into the mechanism / seal causing future issues, so be aware of this.

Along the way to drain outside, often close to the boiler will be a tundish drain which is a funnel shaped (often plastic), drain with an air gap. This air gap serves two purposes. Firstly it allows you to see if water is leaking from your PRV through the tundish. Secondly, the air gap prevents any, potentially contaminated water from drains etc being drawn back in to the boiler, which could cause damage.

Pressure relief valves are clearly there to serve a purpose and function for your central heating system and it wouldn’t be considered very unusual for them to be triggered from time to time in small amounts.

However, if you are seeing a lot of water being released, it is happening regularly or even constantly you likely have a fault that needs to be investigated. Otherwise, as we said above it will compromise the functioning of your central heating boiler system, this may show as an F1 boiler error or Vaillant F75 boiler error, also E119 boiler error and F22 Boiler errors.

Expansion vessels working conjunction / compliment pressure relief valves, in particular in handling smaller variances in water pressure / expansion and contraction. Expansion vessels can be integral or exterior to the boiler itself. There are different designs, shapes colours, etc.

They will often look something like the diagram above. So as the water pressure rises slightly the diaphragm, often made of rubber material, will stretch compressing the air above and then when the pressure falls the reverse happens.

Like pressure relief valves, expansion tanks can fail too, especially if the water and air tight seal between the air and water areas of the tank is broken. If that happens on a expansion vessel then water will enter the area for air and can cause the boiler pressure to drop, sometimes enough to stop the boiler working.

We thought it was worth adding a section about pressure reducing valves which are not the same as pressure relief valves. pressure reducing valves are commonly used in domestic and commercial properties to reduce water pressure into the building all part of the building this can be for a number of different reasons.

Some reasons for installing a pressure reducing valve in a property include – lowering the pressure and therefore strain on your pipework and appliances, sometimes to conserve water, to stop all reduce water hammer problems (knocking on paperwork) or just because people prefer to have lower pressure in their property.

As we said earlier pressure relief valves or safety relief valves can be found in other applications not just on boilers and heating systems. They can be used in devices and machinery where water, air, oil and gas are used. In general they are for safety purposes and to protect the machinery or devices themselves. This is a bit like isolation valves, which have a number of applications.

pressure relief valve can fail for a number of reasons including general wear and tear, broken seals, broken or damaged springs and also because debris has got into the mechanism compromising its effectiveness. most of these problems are resolved by either repairing or replacing the PRV.

if a pressure relief valve fails then not only will it potentially stop your central heating system boiler from working effectively or at all, plus it can mean that water is expelled from your central heating system, leaking out of the outlet pipe which is often found on the wall outside the boiler.

Pressure relief valves can be found number of areas but generally they are located in close proximity to the boiler on the connections between the pressurised central heating boiler itself. Remember that a non-pressurised or vented system will unlikely have one at all. They are often vented to a tank in the loft (or elsewhere) when there is expansion.

A tundish is a funnel shaped, often plastic, connection with a feed (usually from your PRV on your pressurised boiler) with a collection cup at the bottom feeding to a drain. There is a visible air gap between the top and bottom on a tundish to allow you to see any PRV boiler leaks and also to stop any water (from the drain) going back into the boiler system, which could cause issues. If you see your tundish leaking, you could have a problem.

The primary purpose of a safety valve is to protect life, property and the environment. Safety valves are designed to open and release excess pressure from vessels or equipment and then close again.

The function of safety valves differs depending on the load or main type of the valve. The main types of safety valves are spring-loaded, weight-loaded and controlled safety valves.

Regardless of the type or load, safety valves are set to a specific set pressure at which the medium is discharged in a controlled manner, thus preventing overpressure of the equipment. In dependence of several parameters such as the contained medium, the set pressure is individual for each safety application.

At almost every show I attend, I ask a few engine owners and operators if they are satisfied with their new-style safety valves. It would be only a small exaggeration to say that I get just two responses. “I have been using a new-style valve for 15 years and I haven’t had any trouble with it” or “They are junk!” I have cleaned up the second response to spare the reader the unpleasant expletives.

As the result of these conversations, I have attempted to understand why there is such a discrepancy between the reactions to modern safety valves. It now appears that there are a few simple steps we can take when purchasing and installing these valves that might improve our satisfaction with the new-style valves, which are the only ones currently available.

To understand the issues involved in the selection of a safety valve, it is necessary to review the history of safety valves used on hand-fired boilers. I am referring to hand-fired boilers rather than historical boilers because the issues are determined by how the boilers are fired, not how they are constructed or how old they are. The requirements for a modern welded boiler made to the American Society of Mechanical Engineers (ASME) code are the same as for a 100-year-old riveted boiler, if both boilers are hand-fired. The requirements for a safety valve for a boiler that is automatically fired are dramatically different.

With automatic firing, the safety valve’s function as defined by Anderson Greenwood Crosby, a manufacturer of modern safety valves, is to protect life and property if all other safety measures fail. A safety valve on a hand-fired boiler, as defined by ASME almost a hundred years ago, is to give notice of the highest pressure permissible and to give alarm that more water or less fuel is needed. (The evolution of the purpose of the safety valve is summarized in “The Purpose of a Safety Valve,” at the end of this article.)

When hand-fired boilers, such as found on traction engines, steam cranes and locomotives disappeared, most of the boilers that remained were automatically fired. The safety valve manufacturers adapted their designs accordingly. The old-style valves with bottom guided, beveled seats were capable of withstanding vibration and operating near their setpoint, and were replaced by smaller top-guided valves with flat seats.

At the same time, steam system designs were adapted so there was no need to operate within 10 percent of the setpoint of the safety valve. Not all old-style valves had beveled seats, but the ones that didn’t were designed much differently from the modern flat-seated valves.

When I speak of a modern-style safety valve, I am referring to a valve shown in the second diagram of this article’s image gallery. An example of an old-style safety valve can also be seen in the image gallery. Changes in the design of safety valves had a dramatic effect on their capacity in pounds of steam per hour. If a boiler built in 1920 required a valve capable of releasing 1,000 pounds of steam per hour at a pressure of 100 pounds per square inch, it would have been equipped with a 2-inch safety valve. Today, 3/4-inch valves are available to release that much steam at that pressure.

The evolution of the safety valve did not end with the development of the modern, flat-seated valve. In the last 20 years, the design of safety valves has continued to evolve.

In 1985, a new-style 3/4-inch valve set at 150 pounds per square inch could have a capacity of 1,497 pounds of steam per hour. In 2002 this same valve could have a capacity of 1,651 pounds of steam per hour.

From 1914 until 1998, the blowdown allowed by the ASME boiler code was 2 to 4 percent of the setpoint. In 1998, this was changed to allow the blowdown to be as high as 6 percent. Beginning with the 2004 ASME code, there is no limit on blowdown. The code has not required that the amount of blowdown be stamped on a safety valve since 1986.

When I asked owners and operators how well they liked their new-style safety valves, I was not aware of the need to ask about the age of the valve. Instead, I would ask about the amount of blowdown. In almost every case, the people who were satisfied had valves that would blowdown 4 percent or less. It appears that this is almost the same as if I had asked the age of the valve. If the valve were purchased prior to 1998, it would have been set for 4 percent blowdown. If purchased after that date, unless specified otherwise, it would have been set for 6 percent. The change from 4 to 6 percent causes a 50 percent increase in the amount the pressure changes in a boiler each time the valve pops. The resulting increase in the flexing of the components of the boiler may be associated with a corresponding increase in seepage at stay bolts and tubes.

When you order a new safety valve, you will need to provide four pieces of information: the setpoint, the capacity in pounds of steam per hour, the blowdown and the requirement that the valve be stamped with the ASME “V” stamp. If you specify the pipe size, you may get a valve with far too much capacity, as I have already explained. To determine the capacity you need, do not use the capacity stamped on the old valve. First, if the valve has been replaced, it might not be the right capacity. Secondly, the capacity stamped on the valve is probably the capacity of the smallest valve available and might be significantly larger than the required capacity.

Calculating the heating surface of the boiler, in square feet, and multiplying the number by a factor can determine the required capacity. The ASME code requires a factor of 5 pounds of steam per hour per square foot of heating surface. Ohio requires a factor of 7. The heating surface is the area above the grates that is exposed to the fire. This includes the firebox, the tubes and the front tube sheet.

As I talked to many owners, they would offer other comments regarding their valves. One comment I heard from several owners who were satisfied with the new-style valves was that their valves were larger in pounds per hour than the minimum required by the ASME code. I am not certain as to exactly how the capacity affects the operation of the valve. What I do know is the larger the valve, the more force it takes to raise it off its seat. The force of the steam on the boiler-side of the valve cannot overcome the force of the spring in the safety valve until the pressure in the boiler rises to the setpoint of the valve.

When this happens, the valve pops open. It seems the larger diameter, and thus the greater forces, may result in more stable operation of the valve near its setpoint. There is a concern in the boiler code that safety valves should not be so large that water is drawn out of the boiler. It would seem, because of the relatively small size of portable and traction boilers, the pressure would drop so quickly that little water, if any, would be lost. I have seen boilers where the owners have installed modern safety valves of the same pipe size as the old style valves installed by the factory. The capacity of these valves was far greater than I believe anyone would recommend, but I am not aware that they caused water to be discharged from the boiler. It is important to be careful when sizing a safety valve. I suggest owners talk to each other and share their experiences before making such a decision.

If you would prefer to have a top-discharge safety valve, which looks more authentic, shop around. They are available in a few sizes. You might also want to consider specifying that the valve have a non-metallic seat.

When installing a safety valve, do not install any fitting smaller than the inlet to the valve and do not install any kind of valve between the safety valve and the boiler. Examples of what not to do can be seen in the image gallery. Do not use a pipe wrench on a safety valve, it can damage or destroy the valve.

Once you have carefully selected your safety valve and have installed it on your boiler, it is important to verify the setpoint and the blowdown have been set according to your specifications. The first step in this process is to have the accuracy of your steam gauge checked with a dead-weight gauge tester. If your gauge does not agree with the setpoint of your new safety valve do not assume that the gauge is wrong.

Do not use the lifting lever to lift the valve from its seat until the boiler pressure is up to 75 percent of the setpoint of the valve. If the valve is lifted from its seat at a lower pressure, any dirt or foreign material in the valve might not be blown clear of the seat and could damage the seats when the valve closes.

Because new-style safety valves are not designed to be operated within 10 percent of their setpoint, many owners have elected to install the new valve along with an old-style valve. In doing so, the old style valve operates in the range of 5 to 10 percent below the setpoint of the new valve. With this arrangement, the new valve satisfies the code requirements while the old-style valve performs the function for which it was designed. Two possible arrangements can be seen in the image gallery.

In response to complaints from owners of historical boilers who had recently purchased new safety valves, Dean Jagger, Ohio’s chief boiler inspector, requested that the National Board test valves from the manufacturer to determine if the valves complied with the requirements of the ASME boiler code. As a result of these tests the Ohio Department of Commerce issued a safety notice:

“The State of Ohio Boiler Division has been made aware of the fact that some recently purchased Kunkle safety valves, which were assembled by Allied Industries, have been tested by the National Board Testing Laboratory and found not to be in compliance. The tests indicated that the valves blowdown and setpoint pressure settings were out of tolerance as established by Section I of the ASME Boiler Code.”

This may be an indication that all of the problems with modern safety valves are not entirely the result of design issues, but insufficient oversight of manufacturing and quality control processes may also be a factor. A new valve may be “junk” as has been so often alleged.

The errors found by the National Board Laboratories were significant. One of the valves was stamped 165 psi but popped low at 148.8 psi. Another was stamped 150 psi and popped high at 164.5 psi. On the other three valves the pop was consistent with the setting stamped on the valve. The 2001 edition of the ASME Boiler Code specifies that for pressures from 70 to 300 psi the tolerance, plus or minus from the set pressure, shall not exceed 3 percent of the set pressure. The 165 psi valve popped 9.6 percent below the set pressure stamped on the valve, and the 150 psi valve popped 9.7 percent above the set pressure stamped on the valve.

The blowdowns on all of the valves that were tested were out of tolerance. The 2001 edition of the ASME Boiler Code specifies that for pressures from 67 to 250 psi the blowdown shall not be greater than 6 percent of the set pressure. With such a wide range of variations in both setpoint and blowdown, in a sample of just five valves, it seems reasonable to suspect that even greater variations may exist. The results of the tests are shown in the image gallery.

Complaints about quality problems are not limited to the five valves recently tested by the National Board; for example, an engine owner told me of purchasing a new 1-1/4-inch valve stamped 175 psi. When installed on a traction engine, the valve consistently popped at 185 psi and blew down 15 psi. (A pressure of 11.1 psi equals the allowed 6 percent.) The manufacturer told the owner that the valve had been tested properly prior to shipment but accepted it back. The owner had verified the accuracy of the pressure gauge prior to contacting the manufacturer.

The ASME and National Board procedures for safety valves merely confirm the adequacy of the design of the valve and do not assure the adequacy of production and quality control practices. Each boiler owner and operator must carefully confirm the accuracy of the setpoint and blowdown on every safety valve and not rely on the ASME and National Board stamps as assurances of quality. At this time, I have no reason to believe the monitoring of the ASME and National Board requirements at other valve manufacturers and assemblers is any different than what existed at Kunkle and Allied.

One scenario that concerns me is the owner who installs a new safety valve on his boiler and, seeing that the pop does not coincide with the reading on his 80- or 100-year-old gauge, decides that obviously his gauge must be wrong. This is a conclusion I am sure I would have considered when I first began my study of safety valves.

Incorrect settings of safety valves are more likely to be detected when the valves are used on hand-fired boilers than when used on modern boilers. If the controls on a modern boiler are to limit the pressure to 10 percent or more below the setpoint of the safety valve, the valve can be set as much as 10 percent below its rating and the error might not be apparent. Errors above the setpoint also wouldn’t be obvious even if the boiler were operated up to the setpoint stamped on the valve. Also, incorrect setting of the blowdown would not be apparent until the valve had operated.

1909: “The function of the safety valve is two fold: (A) it gives notice of the highest pressure permissible; (B) it gives alarm that more water or less fuel is needed.”

Today: “A PRV (pressure relief valve) is a safety device intended to protect life and property if all other safety measures fail.” – Anderson Greenwood Crosby, 2001 (safety valve manufactuer)

Bob Ferrell, Bob Schuler and Fred Harrison of the National Board of Boiler and Pressure Vessel Inspectors were instrumental in helping to obtain the information from the old editions fo the ASME boiler codes. Their assistance is greatly appreciated.

A fire-tube boiler can be fitted with one or more safety valves on the top of its shell, with each set to open when the boiler reaches its design pressure. Noisolation valvesor restrictions should be integrated between the safety valve(s) and boiler. If the valves are not installed directly onto the boiler shell, the pipework connecting the valves to the boiler must be kept clear of blockagesand water, and this must be confirmed by periodic testing.

Once a safety valve opens, steam is discharged via the exhaust pipe. Exhaust pipes must be designed to encounter as few bends as possible, be as short as possible, to have no reduction in pipe section (no internal pipe diameter reduction), and should lead to asafe point of discharge(typically outside the boiler house).

Water must be drained from the safety valve or exhaust pipework via a drainpipe. Drainpipes may be connected to holes drilled into the lowest section of the exhaust pipework, or, directly to drain holes in the safety valve body; these drains are not to be confused with the blowdown ring locking bolt, if one is fitted.

Where two safety valves are fitted, it is common that one is set just belowthe boiler’s design pressure. It is vital that each safety valve permits the full flow of steam produced when the boiler is operating at maximum capacity i.e. when the boiler is producing the maximum amount of steam it can possibly produce. If safety valves are sized correctly, a boiler can be firing at full capacity without the steam pressure exceeding design limits (because the safety valve(s) relieves pressure at a faster rate than it is accumulated).

There are various types of safety valve, including high lift and improved high lift valves, which use the force of escaping steam to open a winged valve plug to achieve greater steam flow rates. In addition to this, some valves integrate a pistonat the bottom of the spring chamber. The piston has a larger surface area than the valve plug, which leads to the valve opening with a definitive ‘pop’ sound.

Some boiler safety valves include a blowdown ring. The blowdown ring can raise or lower the valve seat ring and is used to control the amount of blowdown through the valve. This ring is locked by a bolt that protrudes through the valve and into the adjusting ring segments.

Boiler safety valves should be fitted with an easing gear (looks like a handle), used, when necessary, to rapidly release boiler pressure. Easing gears can also be used for testing a safety valve, ensuring the spindle has freedom of movement and that the valve operating mechanism functions as intended. Easing gear testing is often not conducted due to operators having difficulty with the valves resealing, but this is generally only the case with valves that are not tested often enough. Actuating the easing gear several times is often all it takes to dislodge debris from the sealing area and allow the valve to seal again. For safe operation, the easing gear handle is usually connected via steel cables to an area neighbouring the boiler.

Like pressure gauges, all safety valves should be stripped, inspected, and calibrated, at least once a year; maintenance usually occurs during statutory inspections. Calibration of each valve should be conducted by a competent person, and any valve adjustment (including the blowdown ring) should be approved and sealed by the authorised inspector. After testing and calibration, all valves should be correctly marked, suitable certificates issued, and accurate records maintained.

An accumulation test can be conducted to ensure a safety valve can relieve over-pressure steam when the boiler burner is operating at maximum capacity. Accumulation testing of safety valves must be repeated after any alterations are made to the boiler e.g. replacement of a safety valve, fuel change, or changes to the control system. If, during an accumulation test, boiler pressure rises by more than 10% of its design pressure, the test must be aborted. Before the boiler is re-tested, amendments must be made to either the safety valve relieving capacity, thesafety valve exhaust pipework, or the boiler’s steaming capacity, to ensure the 10% limit is never exceeded.

Reliefand safetyvalves prevent equipment damage by relieving over-pressurisation of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the set-point pressure.

A relief valve gradually opens as the inlet pressure increases above the set-point. A relief valve opens only as necessary to relieve the over-pressure condition. Relief valves are typically used for liquid systems.

A safety valve rapidly‘pops’ fully openas soon as the pressure setting is reached and will stay fully open until the pressure drops below the reset pressure. The reset pressure is lower than the actuating set-point pressure. The difference between the actuating pressure set-point, and the pressure at which the safety valve resets, is called blowdown. Safety valves are typically used for gas or vapour systems.

A safety relief valve may open fully, or proportionally, once the pressure setting is reached. SRVs may be used for any fluid system (gas, liquid, or vapour).