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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.

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.

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).

Pressure relief valves are used to protect equipment from excessive overpressure. Properly sized relief valves will provide the required protection, while also avoiding issues with excessive flow rates, including: possible valve damage, impaired performance, undersized discharge piping and effluent handling systems, and higher costs.

Many scenarios can result in an increased vessel pressure, and each scenario may result in a different valve size. It is generally recommended to perform multiple case studies to find the most conservative sizing. Some typical cases include:

In any of these scenarios, the pressure will increase until a predetermined relief pressure is reached, at which point the relief pressure valve will open, decreasing the pressure after the turnaround time. The first step in sizing a Relief Valve in ProMax is to determine which scenario you are interested in modeling.

The Relief Valve Sizing in ProMax is performed as a stream analysis. Any stream in ProMax may have one or more Relief Valve Sizing Analyses added, so multiple cases can be studied in a single stream if desired.

There are many different standards for Relief Valve Sizing, each applying different assumptions, thus giving different results. For instance, API 520, one of the most cited standards, assumes both a mechanical and thermodynamic equilibrium, and constant phase properties during relief. Alternatively, the EN ISA 4126 standard accounts for thermodynamic non-equilibrium. ProMax currently supports six different sets of Relief Valve Sizing Standards:

Next, an appropriate relief type must be selected, as sizing depends on the type of relief device selected. The operation of conventional spring-loaded pressure relief valve is based on a force balance: the spring-load is preset to apply a force opposite in amount to the pressure force exerted by the fluid on the other side when it is at the set pressure. When the fluid pressure exceeds the set pressure, the pressure force overcomes the spring force, and the valve opens. Any back pressure (downstream pressure) is additive to the spring force; if this back pressure varies, then the pressure at which the valve opens will vary. Bellows are used to maintain a constant relief pressure despite back pressure variations. Rupture disc relief valves do not reclose after activation; preference should usually be given to reclosing relief devices for both safety and reliability. The most common valve types include:

Balanced Bellows- spring loaded pressure relief valve that incorporates a bellows for minimizing the effect of back pressure on the operational characteristics of the valve.

Pilot Operated- a pressure relief valve in which the major relieving device or main valve is combined with and controlled by a self-actuated auxiliary pressure relief valve (pilot).

The Relief Temperature is determined by which pressure relief scenario you have chosen to model, and should be the temperature of the fluid at the time that the valve is expected to open. ProMax assumes that the Relief Temperature will be the current stream temperature, however, if your particular scenario requires that this be adjusted, it can be overwritten directly in the analysis dialog.

The Over Pressure is usually expressed as a percentage of the Set Pressure. For spring-operated relief valves, a small amount of leakage occurs at 92-95% of theSet Pressure, and sufficient Over Pressure is necessary to achieve full lift. ASME-certified relief valves are required to reach full-rated capacity at 10% or less overpressure.

Once you’ve determined your emergency scenario, and specified the relieving conditions and flowrate, and the appropriate standard, ProMax will calculate the Effective Discharge Area. This value is used to select the appropriately sized Pressure Relief Valve.

Although an orifice is often used to describe the minimum flow area constricted in the valve, the geometry and relief area calculations are more appropriately modeled based on an ideal (isentropic) nozzle. The expression for the mass flux (Gn) in an ideal nozzle is obtained directly from Bernoulli’s equation in the nozzle:

This type of flow is often encountered at metering devices in chemical processing and in relief valve sizing applications where both non-condensable and condensable (flashing) components

This term is an estimation used in sizing pressure relief valves for two-phase liquid/vapor applications when the system has less than 0.1 wt% H2, a nominal boiling range less than 150°F, and

As such, there are multiple methods of approximating the latent heat, and the Relief Valve Sizing analysis follows the methodology of the standards. For example, the API 520 standard defines “latent heat” as