how does a gas oven safety valve work free sample

Most modern appliances have safety features built in, but your gas oven safety valve is arguably the most important. If an electrical appliance malfunctions, it can cause a fire, but a misfiring gas oven could potentially blow up your house. You don"t ​really​ need to know how the safety mechanism works to use your oven, but you may find that it gives you some extra peace of mind.

Broadly speaking, there are two ways a built-in safety mechanism can work. One option is that it remains "open" by default and to shut off if certain conditions are met. That"s how fuses and circuit breakers work in an electrical circuit: Ordinarily, the electricity is free to flow, but if the current grows too large, the fuse or breaker will blow and cut off the circulation of electricity.

The other option is for your safety mechanism to be "closed" by default and allow a device to operate only when the correct conditions are met. That"s how a gas oven safety valve works. Gas ordinarily is prevented from flowing, and if the valve is working correctly, it opens only when you want to light your oven.

Many gas stoves use what"s called a "hot surface igniter," a bar or element (similar to the ones on your stovetop) that gets hot enough to ignite the gas on contact. Gas oven safety valves on stoves with this type of ignition system take a couple of different approaches.

In one approach, a bimetallic strip operates the valve. It harnesses a simple scientific principle: Metals expand and contract at different rates when they"re heated and cooled. If you bond two suitable metals together in one strip, that strip will flex to a predictable degree as the temperature goes up and down. Wall-mount thermostats often use this principle, as do analog oven thermometers and the thermometer in the lid of your gas grill.

As appliance-repair website PartSelect explains, turning on your gas oven causes electricity to flow into the heating element of your hot surface igniter. As the igniter heats up, it warms a bimetallic strip inside your gas oven safety valve. When the igniter reaches its operating temperature, the bimetallic strip opens the valve and allows the gas to flow, igniting as it crosses the heated surface.

One intriguing thing about electricity is that a change in temperature can affect how well it passes through certain materials. For example, a lot of research revolves around ​superconductors​ – materials that offer very little resistance to an electrical current – but superconductors typically must be heavily chilled to work.

According to heating-equipment vendor Anglo Nordic, gas oven safety valves use a variation of that principle to operate. In these stoves, the flow of electrical current through the hot surface igniter becomes the control mechanism. The igniter"s bar is made of a material that offers less and less resistance to electricity as it heats. When it reaches the temperature required to ignite the gas, its resistance becomes low enough to trip the safety valve and open the flow of gas.

More modern ranges use an electrical igniter. When you turn on your oven, the gas begins flowing immediately, and it sends an electrical current to a piezo electric igniter. The current makes the igniter spark (like the manual igniter on your gas grill) and lights the oven"s burner. In this case, the safety valve works in the opposite way: An electronic sensor checks for the heat caused by ignition after a few seconds, and if it"s absent, it will close the valve and shut off the flow of gas.

It"s worth pointing out that not all gas ovens have a safety valve in the conventional sense. Older stoves simply use a pilot light, a small but constant flow of gas, which, in turn, feeds a small, candle-like flame. You essentially ​are​ the safety mechanism in this system: It"s up to you to check that the pilot is lit. When you turn on the gas manually, the small pilot flame ignites the main flame. It"s a mechanically simple system, which makes it durable, and for that reason, you"ll still see it used on commercial restaurant ranges, which must stand up to decades of heavy use.

Pilot ignition systems use a flame sensing element to sense whether the pilot is lit and the safety valve can open. The sensing element sits right in the pilot flame.

Just exactly where the sensor sits in the pilot flame is important. (See figure 6-A) If the sensing bulb is not in the right part of the flame, or if the pilot is adjusted too low or too high, it will not get hot enough and the safety valve will not open.

When two dissimilar metals (for example, copper and steel) are bonded together electrically, and then heated, they generate a tiny electrical current between them. The voltage is very small, measured in millivolts. This is the basis for a millivolt oven ignitor system. All that"s needed is a safety valve that will sense this tiny voltage and open the valve if it is present. If the pilot is out, there is no millivoltage and the safety valve will not open. See figure 6-B.

If the burner in a millivolt system will not start, typically the problem is the gas valve. Occasionally the problem might be the pilot generator or thermostat. The thermostat in these is just a temperature-sensitive on/off switch. To test, turn it on and test for continuity.

If that doesn"t work, we have a minor dilemma in determining whether the problem is the pilot generator or the safety valve. The dilemma here is that the voltages are too small to be measured with standard equipment. VOM millivolt adaptors cost nearly as much as the pilot generator itself. And the safety valve, which is usually the problem, costs twice as much as the pilot generator. So usually you just replace either or both of them. But don"t forget they are electrical parts, which are non-returnable. What I recommend is just to replace the gas valve first; that usually will solve the problem. If not, replace the pilot generator. You just ate a gas valve, but trust me, you"d have bought one sooner or later anyway.

When installing the pilot generator, screw it into the safety valve finger tight, plus 1/4 turn. Any tighter than that and you can damage the electrical contacts on the valve.

In some systems the sensor is a liquid-filled bulb, with a capillary to the safety valve or flame switch. When the liquid inside heats up, it expands and exerts pressure on a diaphragm, which opens the valve or closes the switch.

It is important to know that these sensor bulbs do not cycle the burner on and off to maintain oven temperature. That is the thermostat"s function. It has a sensor bulb too, but it senses oven temperature, not pilot flame. The only function of these pilot sensing elements is to prevent gas flow to the burner if the bulb does not get hot enough to assure burner ignition.

In flame switch systems, hydraulic pressure from the capillary physically closes the switch, which completes an electrical circuit to the safety valve. The safety valve is electrical and operates on 110 volts. See Figure 6-D. If the pilot is out, the flame switch does not close and the 110 volt heating circuit is not complete, so the safety valve will not open.

Some of these direct-pressure (hydraulic) systems use a two-level pilot. The pilot stays at a very low level; not even high enough to activate the safety valve. This is called the constant pilot, or primary pilot. Gas for the primary pilot may come from either the thermostat or directly from the gas manifold.

When the thermostat valve is turned on, the pilot flame gets bigger, heating the sensor bulb, which activates the safety valve (hydraulically) and the burner ignites. This is called the heater pilot, or secondary pilot. Gas for the secondary pilot comes from the oven thermostat itself.

When the gas oven reaches the correct temperature setting, the thermostat drops the pilot flame back to the lower level, the safety valve closes and the burner shuts off. See figure 6-E.

The sensing bulb needs to be sitting right in the hottest part of the flame as described in section 6-2. If you don"t have a good strong pilot (secondary pilot, in two-level systems) that engulfs the pilot sensing bulb with flame, try cleaning the pilot assembly and sensor bulb as described in section 6-5. If that doesn"t work, replace the pilot assembly.

If you do have a good strong pilot that engulfs the pilot sensing bulb with flame, then odds are that the sensing element and/or whatever it is attached to are defective. If it is a flame switch, replace the flame switch. If it is a safety valve replace that.

In a two-level pilot system, remember that the main oven thermostat supplies the secondary pilot with gas. So if you cannot get a good secondary pilot the problem may be the pilot assembly, or it may be the thermostat. If you do get a good secondary pilot, you"re back to the sensing bulb and safety valve.

Spark ignition systems use a spark module to generate a pulsing, high-voltage spark to ignite the gas. The spark module is an electronic device that produces 2-4 high-voltage electrical pulses per second. These pulses are at very low amperage, measured in milliamps, so the risk of shock is virtually nil. But the voltage is high enough to jump an air gap and ignite gas. The spark ignition module is usually located either under the cooktop or inside the back of the stove. The same module is used for both the surface burner ignition and the oven burner ignition.

However, the spark is not certain enough to light the oven burner, and the gas flow is too high, to rely on the spark alone. Remember, in an oven, before the safety valve opens, you need to be assured of ignition. So the spark ignites a low-gasflow pilot, and then the safety valve opens only when the pilot is lit.

This is the same two-level pilot system described in section 6-2(b), with a few important exceptions. The constant or primary pilot does not stay lit when the oven thermostat is turned off. It does, however, stay lit the whole time the oven thermostat is turned on.

When the gas oven is turned on, a switch mounted to the oven thermostat stem signals the spark module. These are the same switches as shown in section 5-3.

The flame is positioned between the spark electrode and its target. The pilot flame actually conducts electricity. So when the pilot flame is burning, electricity from the spark electrode is drained off to ground, and sparking stops. If the pilot quits, sparking resumes.

When the thermostat calls for more heat in the oven, the heater or secondary pilot increases the size of the pilot flame, which heats the sensing bulb, which opens the safety valve and kicks on the burner.

Yup, this ol" boy"s got it all. Spark ignition, a pilot, a flame switch and TWO - count "em - TWO safety valves; one for the pilot and one for the burner. (Figure 6-H)

The operation is actually simpler than the diagram looks. When you turn on the oven thermostat, a cam on the thermostat hub closes the pilot valve switch. This opens the 110 volt pilot safety valve and energizes the spark module, igniting the pilot. As in the other spark system, the pilot flame provides a path that drains off the spark current, so the ignitor stops sparking while the pilot is lit. As long as the oven thermostat is turned on, the pilot valve switch stays closed, so the pilot valve stays open and the pilot stays lit.

When the pilot heats the pilot sensing element of the flame switch, the flame switch closes. This completes the 110 volt circuit to the oven safety valve, so the valve opens and the burner ignites.

When the oven temperature reaches the set point of the thermostat, the thermostat switch opens, breaking the circuit and closing the oven safety valve, and shutting off the burner.

Now that you know how the system works, first look to see what is not working. When the oven thermostat is on, and there isn"t a pilot flame, is the electrode sparking? Is there spark, but no primary pilot? Is the primary pilot igniting, but not the secondary? Is there sparking after the thermostat is shut off?

(The pilot may or may not light, but the main burner is not lighting) Remember that the thermostat supplies the pilot with gas in these ovens, and only when the thermostat is on. So if you don"t have a primary and secondary pilot flame, odds are the problem is the pilot orifice or oven thermostat. Try cleaning the pilot assembly and sensor bulb as described in section 6-5. If that doesn"t work, adjust the secondary flame a little higher. If that doesn"t work, replace the pilot assembly.

If you do have a good strong secondary pilot that engulfs the pilot sensing bulb with flame, then odds are that the oven safety valve (or flame switch, whichever is attached to the pilot sensing bulb in your system) is defective. Replace the defective component.

Something is wrong with the high-voltage sparking system. If you are in a hurry to use your oven, you can turn on the oven thermostat, carefully ignite the primary pilot with a match and use the oven for now; but remember that the minute you turn off the thermostat, the pilot goes out.

Are the cooktop ignitors sparking? If so, the spark module is probably OK. What typically goes wrong with the sparking system is that the rotary switch on the valve stops working. Test continuity as described in section 5-3(a). If that isn"t the problem, check the electrode for damage and proper adjustment. The spark target (the nearest metal to the electrode) should be about 1/8″ to 3/16″ away from it, (about the thickness of 2-3 dimes) and directly across the primary pilot orifice. Replace or adjust the electrode as appropriate. When replacing, make sure you get the right kind of electrode (there are several) and do not cut the electrode lead; follow it all the way back to the spark module and plug the new lead into the proper spark module terminal.

Usually the ignition switch has gotten some moisture in it and it is shorting. To test, pull the lead off the switch and see if the sparking stops. If so, the switch is bad. Replace it.

Remember that these switches are on 110 volt circuits. If you get too fast and loose with pulling these leads off to test them, you might zap yourself.

As a design engineer responsible for developing and specifying boilers, dryers, furnaces, heaters, ovens and other industrial heating equipment, you face a daunting labyrinth of standards and industry regulations. Regulatory bodies sound a bit like alphabet soup, with acronyms like UL, FM, CSA, UR, AGA, ASME, ANSI, IRI, CE and NFPA tossed about. This article will help explain a common task for many thermal processing equipment specifiers: meeting the requirements of key codes — including Underwriters Laboratories (UL), Factory Mutual Insurers (FM) and the National Fire Protection Association (NFPA) — for safety valve equipment used in process heating applications.

Key to designing safety into your fuel train configurations are familiar technologies such as safety shutoff valves and vent valves as well as visual-indication mechanisms and proof-of-closure switches.

Your design skills come into play with how you take advantage of the wide range of products available. You can mix and match solenoid and safety shutoff valves — within designs from catalytic reactors to multi-zone furnaces — to create easily installed, cost-effective solutions that comply with all necessary standards. (See table.)

Make sure, however, that you start with a good grasp of valve element fundamentals. For example, examining a proof-of-closure (POC) switch underlines how reliably modern valves can ensure combustion safety. The POC unit provides an electrical contact interlocked with the controller safety circuit. In a typical design, the switch is located at the bottom of the valve, positioned to trace the stroke of the valve disc. When the disc seal reaches the fully closed position, it triggers the mechanism to push down on the contact, closing it and triggering the unit’s visual indicator to show open or closed status. As a result, the operator can act with full confidence in situations where it is critical that a safety valve be safely closed.

To provide ease of installation, many users prefer valves with modular capabilities. For example, to reduce mounting complexity, you can choose modular gas safety shut-off valves — combining a solenoid valve with an electrohydraulic motorized valve for a compact double-valve footprint, a slow-open feature and high flow rates. An accompanying actuator can provide on/off or high/low/off firing rates as well as visual indication and proof of closure for compliance with most industry standards.

Also, you may want to look for valves that include useful features such as pipe taps, which can facilitate accurate pressure readings and leakage testing.

Knowing your valve choices — and how they meet given codes and standards — can reduce the time required for design and production while facilitating compliance. This results in safer, more efficient and cost-effective heating process installations.

On a pilot ignition oven, there is a pilot which is an actual gas flame (although very small) in the oven. What happens in this system is that the pilot stays lit all the time and when the oven thermostat is turned on, the pilot flame extends (gets bigger) to envelope the thermocouple bulb of the oven safety valve. The thermostat controls gas flow to both the pilot and to the oven safety valve. Once the oven safety valve"s thermocouple senses appropriate heat, the safety valve opens to allow gas flow to the oven burner where the pilot flame ignites the gas.

On a pilot ignition system the pilot flame must be lit for the oven burner to receive gas. If the pilot will not stay lit, there may be a problem in the oven thermostat (which supplies gas flow to the pilot) or the gas tubing leading to it and/or the pilot itself may be plugged with cobwebs, grease, etc.

If the pilot IS lit but the oven burner is not lighting, the pilot flame will need to be inspected as the thermostat is turned on. If the pilot is not extending when the thermostat is tuned to ON, the oven thermostat may be defective. If the pilot is extending but the oven gas valve is not opening, that oven safety valve may be defective or its sensor bulb may be out of position on the pilot or just dirty causing it to not sense the proper temperature of the pilot flame. A dirty pilot can also cause a reduced size pilot flame so the oven safety valve can not sense the proper temperature to open.

This older type of electronic ignition system was popular in the mid 70"s and is for the most part no longer used but ranges/ovens employing it may still be in service. It was a fairly reliable setup.

On this style of ignition system there is a constantly burning pilot similar to the pilot ignition system described above but the pilot flame does not change in size as the control is turned on. This system uses a "flame switch" to detect that the pilot is lit instead of a safety valve like on the pilot ignition system. The thermostat on such a system is just electrical and does not alter gas flow to the oven.

The pilot flame should heat the tip of the flame switch"s sensor bulb to a red colour. If the flame switch detects sufficient heat on its sensor bulb at the pilot, it will close an electrical contact in its base. Once the thermostat is turned on, current will be allowed to flow through the flame switch to the oven valve to allow it to open. If the flame switch"s sensor is not heated sufficiently, the electrical contacts of the flame switch would remain open preventing any power from reaching the oven valve which in turn would prevent gas from flowing to the oven burner.

The pilot must be lit before electrical current will flow through the flame switch to the oven gas valve to allow gas to the oven burner. The flame switch"s contacts must be closed when its sensor is heated by the pilot. The thermostat"s contacts must close when turned on to allow current to flow to the rest of the oven system. A fault in any of those functions will prevent gas from flowing to the oven burner as a safety precaution.

This is the most popular system currently used for ovens and is comprised of a control mechanism (whether thermostat or electronic control), the oven ignitor and an oven gas valve.

What happens in this style ignition system is that the thermostat or electronic control switches power to the oven ignitor and gas valve circuit which are connected in series (one after the other). As power flows through the ignitor it heats and draws current (measured in amperage). Once the oven ignitor draws a specific amount of current the oven valve opens to allow gas to flow to the oven burner where the glowing hot ignitor (glow bar) ignites it. Power must continually flow through the ignitor and oven gas valve for gas to be released into the oven burner to create a flame. Once the set temperature is achieved the control stops all power to the ignition circuit which causes the ignitor to dim and the oven gas valve to close, stopping any burner flame. Cycling on and off continues to maintain the specific temperature the control is set for.

It should usually only take in the area of 30-90 seconds for the oven ignitor to reach the proper resistance to allow the proper amperage to reach the gas valve to open it and for the ignitor to ignite the gas at the oven burner.

Many ovens use a single oven burner in which case they only have a single gas valve and ignitor. The same burner is used for both bake and broil functions, the broil usually being in the drawer area below the oven. Higher-end models may have a separate bake and broil burner. On such a system there will be two ignitors, one for each burner. They may also employ a "dual" gas valve (see illustration above) instead of using a separate valve for each burner.

On oven models utilizing a "dual" gas valve, both the bake and broil gas and ignition systems should be considered totally independent of each other. Even each side of the dual gas valve should be evaluated as if each side was a separate "single" gas valve. Just because one side of a dual gas valve might work does NOT mean the other side of the valve should work too. Each is a separate mechanism inside a common housing.

As long at the ignitor is being powered, the control system (hydraulic thermostat or electronic control) is doing its job. The ignitor, while glowing, may not be allowing the proper amount of current to flow through it to the oven gas valve for it to open. Proper testing (see the link at the bottom of the page) requires an ammeter to check the current flow through the ignition circuit to the oven bake/broil valve.

Weak oven ignitors can glow but not achieve an adequate resistance to allow the correct current to flow to the gas valve for it to open. They can also glow but not be quite hot enough to ignite the gas immediately. See "Mini-Explosions" below for more on the latter.

If the ignitor ages and gets "weak", it is possible for it to glow and generate some warmth in the oven but the oven burner never actually come on to get the oven up to proper temperature. It is also possible for the oven burner to light and heat once but never cycle back on again afterward. The heat generated by the ignitor may keep the oven warm for a while until it is noticed that the oven isn"t heating "right" any longer.

Power must flow from the control system to the ignitor and through the oven valve. If there is any break in that circuit, the ignitor will not glow and thus the gas valve not open. The ignitor not glowing could be caused by the ignitor or the gas valve being open (infinite resistance - no continuity). If both have continuity (ie. at least some resistance) and the ignitor is still not glowing, a problem in the control system or the rest of the electrical circuit would need to be investigated.

An ignitor and oven valve having continuity does not necessarily that they are good, just that they"re not electrically open, which is only one way that they might fail. If either has NO continuity at all (ie. infinite resistance) they are likely defective and need to be replaced.

In the latter case, the oven should be removed from use until the condition can be looked into and corrected. DO NOT continue to use a gas appliance in such a condition!

It can sometimes happen that the ignitor is just on the verge of allowing the correct amperage to the oven valve, but not quite. In such a case the oven valve can repeatedly cycle open and close quickly, creating a "pulsating" or sputtering burner flame. Testing for proper amperage draw of the ignition circuit is needed to determine if a "weak" oven ignitor may be responsible instead of an internal problem with the oven valve itself.

*There is NO voltage or resistance test that will help to diagnose the cause of the problem at this point. Only an amperage test is relevant. An ignitor with a white or beige colored rectangular base should draw 3.2 to 3.6 amps or it is defective. A round base or powder-blue colored rectangular base ignitor should draw 2.5 to 3.0 amps.

Instead, when the oven thermostat is turned on, gas flows to the oven safety valve and also to the oven"s pilot, which gets lit via a spark. Once the pilot is lit and the safety valve"s sensor bulb senses that pilot flame, that valve will then open up to allow gas to flow to the oven burner where the pilot flame ignites the burner.

Whirlpool has introduced a new variation on the spark ignition system. In this system there is no "pilot", instead the spark directly ignites the gas burner (hence the name) and an additional electronic control monitors its performance.

When the range control calls for either bake or broil, the DSI module electronically checks both gas valves" solenoids for continuity. If the checks fail, the module will turn the oven off or lock it out. If the checks are successful, the module will power the appropriate gas valve so it can open and also initiate sparking at the burner ignitor. Both the bake and broil ignitors will spark simultaneously.

If no flame is present at the burner, the DSI module will allow the ignitor to spark for 4 seconds. A 30 second pause will then occur, after which time it will attempt to light the burner again. If gas still fails to ignite, the microcomputer will then lock the system out.

Once gas has ignited, the flame sensing circuitry will monitor the flame at the burner as long as it is powered to make sure that it is present. If at any point no flame is detected, the DSI module will lock out the system.

On such a system the oven will not light for 30 seconds after power is restored to the appliance. Also, if no ground is found in the power connection to the range (or just at the DSI control), the oven will light once but them turn off and be "locked out".

Whirlpool"s DSI system will likely only be found on Whirlpool"s brands of appliances such as Whirlpool, KitchenAid, Roper and Inglis as well as some Whirlpool built Kenmore ranges and ovens.

Aside from the electronic ignition systems, the basic premise for the safety pilots used on gas logs is the same as those used for decades with wall heaters,

You turn the control knob to the pilot position, push the knob in, light the pilot with a match, then hold the knob down for 30 seconds or so until the pilot stays lit by itself. Once the pilot light

will stay lit on its own, you can then move the knob to the "On" position. For manually operated gas logs such as the one pictured to the right, this will turn the logs on. For remote controlled

If you cannot get the pilot to light at all, meaning you hold the button down in the pilot position and hold a match up to the pilot and nothing happens,

then either the valve is bad or something is abstructing the gas from coming into or going through the valve. If you can get the pilot to light with a match, but it will not stay

lit on its own after holding down the knob for 30 seconds, then put the knob back into the off position, wait 5 minutes and try again. If it still will not light, then something is definately wrong and

This page is not meant to be a trouble-shooting guide for gas logs, but in general, if you cannot get your safety pilot to stay lit, It could be that the pilot flame needs adjustment, the

thermocouple has gone bad and needs to be replaced, the entire valve has overheated and must be replaced, or something is abstructing the gas line. In any case, it is probably time to seek the assistance

of a professional. Any plumber or heating and air conditioning service man who deals with gas appliances with a safety pilot should be able to help you.

Although most of us have learned how to light one of these things at some time or other, few of us have any idea as to how this ingenious little safety system actually works.

So here is a brief, but hopefully useful explanation of how gas log safety pilots work so you can decide if it is something that you want or need. You may also find this information

Gas Logs that have a safety pilot have a valve body that is attached directly to the burner. This valve body that has 2 separate valves inside that control the gas:

The thermocouple is the ingenious device that makes the whole system work. The physical properties of the thermocouple are such that it actually generates electricity when there is a great

The electricity from the thermocouple is used to power an electromagnet that holds the pilot valve open, thus allowing the pilot to stay lit by itself. The amount of electricity needed

then the electromagnet no longer functions and the pilot valve shuts. When you turn the knob to the pilot position and push it in, you are in fact manually opening the valve to the

buy whatever means the remote control uses. In the case of a remote controlled valve, some will have a battery operated device that opens and closes the valve to the main burner, thus turning the logs

on and off. More sophisticated systems (called variable flame remotes) will have a battery operated motor attached to the flame adjustment knob that will allow you to adjust the flame height as well.

The main burner valve is designed such that if the pilot valve is closed, no gas can flow through the main valve, even if you have it in the on position.

properly, the system is operational and gas can then be allowed to pass through the main burner valve. If the pilot light gets turned off or blown out (or in some cases gets too hot),

When the main burner is turned on, either by a remote controlled unit or by manually turning a knob, gas flows through the main valve and comes out the holes in the burner.

The flame from the safety pilot is positioned just above the first several holes in the main burner, so when gas flows out of the main burner and reaches the safety pilot, it automatically ignites.

So again, if the safety pilot is not lit (or for some reason the safety pilot gets blown out), the system automatically closes both valves so that no gas will

This prevents the system from allowing gas to flow freely into your home at any time in the event that the safety pilot blows out, or someone turns on the gas to your fireplace

If your gas range is not working correctly, you should check the gas pressure regulator shut-off valve. The factory default setting for the gas pressure regulator is in the "ON" position but may have been turned to the "OFF" position during handling or transportation. When the shut-off valve is on the "OFF" position, gas will flow to the cooktop burners but will not provide a gas supply to the oven.

You can check if the shut-off valve if you can slide the range out from the cabinet. If you are unable to slide the range out, we recommend consultation with a local certified technician.

Verify the pressure regulator shut-off valve is in the open position. The pressure regulator is located at the back of the range. Make sure that the shut-off valve lever is in the "On" position (see illustration below).

NOTE: If the range is hard piped, you will not be able to slide it out from the cabinet if it connected with a flexible supply line, take care not to over-extend the supply line. The main gas valve will usually be at the end of a fixed pipe and connect to the pressure regulator with a flexible supply line. Take care not to kink or pinch this flexible pipe.

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In order to ensure that the maximum allowable accumulation pressure of any system or apparatus protected by a safety valve is never exceeded, careful consideration of the safety valve’s position in the system has to be made. As there is such a wide range of applications, there is no absolute rule as to where the valve should be positioned and therefore, every application needs to be treated separately.

A common steam application for a safety valve is to protect process equipment supplied from a pressure reducing station. Two possible arrangements are shown in Figure 9.3.3.

The safety valve can be fitted within the pressure reducing station itself, that is, before the downstream stop valve, as in Figure 9.3.3 (a), or further downstream, nearer the apparatus as in Figure 9.3.3 (b). Fitting the safety valve before the downstream stop valve has the following advantages:

• The safety valve can be tested in-line by shutting down the downstream stop valve without the chance of downstream apparatus being over pressurised, should the safety valve fail under test.

• When setting the PRV under no-load conditions, the operation of the safety valve can be observed, as this condition is most likely to cause ‘simmer’. If this should occur, the PRV pressure can be adjusted to below the safety valve reseat pressure.

• Any additional take-offs downstream are inherently protected. Only apparatus with a lower MAWP requires additional protection. This can have significant cost benefits.

Indeed, a separate safety valve may have to be fitted on the inlet to each downstream piece of apparatus, when the PRV supplies several such pieces of apparatus.

• If supplying one piece of apparatus, which has a MAWP pressure less than the PRV supply pressure, the apparatus must be fitted with a safety valve, preferably close-coupled to its steam inlet connection.

• If a PRV is supplying more than one apparatus and the MAWP of any item is less than the PRV supply pressure, either the PRV station must be fitted with a safety valve set at the lowest possible MAWP of the connected apparatus, or each item of affected apparatus must be fitted with a safety valve.

• The safety valve must be located so that the pressure cannot accumulate in the apparatus viaanother route, for example, from a separate steam line or a bypass line.

It could be argued that every installation deserves special consideration when it comes to safety, but the following applications and situations are a little unusual and worth considering:

• Fire - Any pressure vessel should be protected from overpressure in the event of fire. Although a safety valve mounted for operational protection may also offer protection under fire conditions,such cases require special consideration, which is beyond the scope of this text.

• Exothermic applications - These must be fitted with a safety valve close-coupled to the apparatus steam inlet or the body direct. No alternative applies.

• Safety valves used as warning devices - Sometimes, safety valves are fitted to systems as warning devices. They are not required to relieve fault loads but to warn of pressures increasing above normal working pressures for operational reasons only. In these instances, safety valves are set at the warning pressure and only need to be of minimum size. If there is any danger of systems fitted with such a safety valve exceeding their maximum allowable working pressure, they must be protected by additional safety valves in the usual way.

In order to illustrate the importance of the positioning of a safety valve, consider an automatic pump trap (see Block 14) used to remove condensate from a heating vessel. The automatic pump trap (APT), incorporates a mechanical type pump, which uses the motive force of steam to pump the condensate through the return system. The position of the safety valve will depend on the MAWP of the APT and its required motive inlet pressure.

This arrangement is suitable if the pump-trap motive pressure is less than 1.6 bar g (safety valve set pressure of 2 bar g less 0.3 bar blowdown and a 0.1 bar shut-off margin). Since the MAWP of both the APT and the vessel are greater than the safety valve set pressure, a single safety valve would provide suitable protection for the system.

However, if the pump-trap motive pressure had to be greater than 1.6 bar g, the APT supply would have to be taken from the high pressure side of the PRV, and reduced to a more appropriate pressure, but still less than the 4.5 bar g MAWP of the APT. The arrangement shown in Figure 9.3.5 would be suitable in this situation.

Here, two separate PRV stations are used each with its own safety valve. If the APT internals failed and steam at 4 bar g passed through the APT and into the vessel, safety valve ‘A’ would relieve this pressure and protect the vessel. Safety valve ‘B’ would not lift as the pressure in the APT is still acceptable and below its set pressure.

It should be noted that safety valve ‘A’ is positioned on the downstream side of the temperature control valve; this is done for both safety and operational reasons:

Operation - There is less chance of safety valve ‘A’ simmering during operation in this position,as the pressure is typically lower after the control valve than before it.

Also, note that if the MAWP of the pump-trap were greater than the pressure upstream of PRV ‘A’, it would be permissible to omit safety valve ‘B’ from the system, but safety valve ‘A’ must be sized to take into account the total fault flow through PRV ‘B’ as well as through PRV ‘A’.

A pharmaceutical factory has twelve jacketed pans on the same production floor, all rated with the same MAWP. Where would the safety valve be positioned?

One solution would be to install a safety valve on the inlet to each pan (Figure 9.3.6). In this instance, each safety valve would have to be sized to pass the entire load, in case the PRV failed open whilst the other eleven pans were shut down.

If additional apparatus with a lower MAWP than the pans (for example, a shell and tube heat exchanger) were to be included in the system, it would be necessary to fit an additional safety valve. This safety valve would be set to an appropriate lower set pressure and sized to pass the fault flow through the temperature control valve (see Figure 9.3.8).