PSV design requires a thoughtful approach that accounts for many variables. This might seem overwhelming but a good PSV design is achieved in 5 steps:

• Valve Capacity
• Set pressure
• Inlet piping
• Safe Location
• Discharge piping

The goal of this article is to give a high level understanding of each of these factors.

Valve capacity

The valve capacity is determined by two factors:

• The necessary capacity
• The certified capacity

The necessary capacity is determined by a blow down and relief study. The main standards used as a guide to perform these studies are:

• API 521 for pressure vessels
• ISO 28300 for atmospheric (storage) tanks

Both standards cover a wide variety of possible scenarios for which a safety valve could be designed. It is up to the user to determine which scenarios are applicable. Some examples of the most common scenarios are:

• External fire
• Blocked outlets
• Tube rupture (in heat exchangers)
• (partial) Utility failure
• Control loop / valve failure

Both standards give additional information on how these scenarios should be approached and give background information about the calculation methods.

The certified capacity is determined by the valve itself. It is an inherent characteristic of the valve at the opening pressure of the valve. Usually, the valve is sized for the governing case: this is the scenario with the highest necessary capacity. Be aware that the actual flow through the valve should match the certified capacity as much as possible. For most spring operated valves, if the actual flow during a scenario is significantly lower than what the valve was sized for, the valve won’t work properly. Oversizing a PSV may lead to hammering, which can damage the valve. However, multiple solutions are available.

In Conclusion.

A PSV is sized to match the governing case, which is determined during a blowdown & relief study. However, some consideration is necessary if the PSV is to cover multiple scenarios with a wide variety of flows, to prevent damaging the PSV.

Further reading:

How to perform blowdown & relief studies (coming soon)

How to size PSVs (coming soon)

PSV hammering: occurrence and mitigation (coming soon)

Set Pressure

The set pressure depends on the type of vessel that needs to be protected:

• A pressure vessel,
• An atmospheric (storage) tank.

In case of a pressure vessel, the maximum set pressure is the MAWP of the vessel. The allowable accumulation above the set pressure – which is necessary for the valve to open to reach its certified capacity – is determined by the vessel construction code. The pressure at which the valve reaches its certified capacity is called the opening pressure.

Opening pressure = set pressure + accumulation.

The following list shows the maximum allowed opening pressures. The MAWP = 100 %.

• ASME
  • 110 % for all scenarios (1 PSV)
  • 116 % for all scenarios (multiple PSVs)
  • 121 % for fire case
• PED
  • 110 % for all scenarios (1 PSV)
  • 110 % for all scenarios (multiple PSVs)
  • 110 % for fire case

In case of an atmospheric (storage) tank, the maximum opening pressure is equal to the MAWP of the tank. No accumulation above the MAWP is allowed. This is true for over pressure and vacuum pressure, e.g. in case of breather valves.

In Conclusion.

The explanation above gives the boundary conditions, but set pressure can be chosen freely between 0 and the mentioned maximums. For example, some safety valves require more than 10 % accumulation above the set pressure to achieve certified capacity. In that case, the set pressure should be chosen low enough, so that the opening pressure never exceeds the maximum allowed pressure defined by the construction code. Keep in mind, a higher set pressure results in a smaller valve, and allows for longer inlet- and outlet piping.

Further reading:

Set pressure – ASME – PED

Inlet piping

The inlet pressure drop is the pressure drop in the inlet piping of the PSV. It is determined at certified capacity. If the inlet pressure drop is too high, it will cause the PSV to chatter. Chatter will cause damage to the PSV and prevent it from operating as intended.

As a general rule of thumb, for most spring operated PSVs, the inlet pressure drop should be < 3% of the set pressure.

This rule might not always apply, especially for weight-loaded PSVs. Always check with the vendor of the valve.

In Conclusion.

For most PSVs, the inlet pressure drop should be < 3 % of the set pressure.

Further reading:

PSV blowdown vs 3% pressure drop (coming soon)

Inlet piping: problems and solutions (coming soon)

How to calculate pressure drop in pipes (coming soon)

Safe location

The effluents of a PSV should always be vented to a safe location. The disposal of effluents from a PSV is subject to many variables. Two main categories can be distinguished:

• Vent to disposal system
• Vent to atmosphere

The choice depends on the physical properties of the effluent, the location of the valve (exit piping), the environment, etc.

Common disposal systems are flares, blowdown systems, incinerators and vapor control systems. Disposal systems are usually designed as one system for multiple relief effluents. This is a complex study which requires input from many disciplines. The general steps when conducting the study are usually:

• Perform a blowdown & relief study to determine the relieving loads discharging into the disposal system.
• Determine which relieving loads will be discharging to the disposal system at the same time. In other words, which scenarios are jointly affected by the same contingencies? External fire is an example of one contingency that may cause multiple PSVs to open on multiple vessels.
• Determine the maximum load into the disposal system from these scenarios.
• Calculate the design load for the disposal system.

Quickly it becomes quite clear that the disposal system may introduce significant back pressure on a PSV, so the design process may become iterative.

If local environmental regulations allow for it, atmospheric discharge is an economical and reliable solution. Careful attention is necessary to establish whether the discharge to atmosphere doesn’t impose any danger to personnel or the environment. The physical properties of the fluid will determine whether the discharge could cause flammable vapors, expose personnel to toxic substances, damage the unit or surroundings with corrosive products, cause air pollution, cause excessive noise, etc. However, these boundaries can be checked quite easily.

In Conclusion.

A PSV should always discharge to a safe location to protect personnel and the environment.

Further reading:

PSV safe location: a step by step guide (coming soon)

Discharge piping

The discharge piping of a PSV causes back pressure on the valve. Back pressure consists of two elements: constant superimposed back pressure and built-up back pressure.

The constant superimposed back pressure increases the set pressure of the valve, and should be accounted for when selecting the set pressure. If the constant superimposed back pressure is significantly high, it may cause the valve to reach subsonic flow conditions. This has an impact on the certified capacity of the valve, so always check with the vendor.

The built-up pressure is the pressure drop in the discharge piping of the PSV. It is determined at certified capacity. If the pressure drop is too high, it may prevent the valve to reach full capacity. It may also lead to chattering. Chatter will cause damage to the PSV and prevent it from operating as intended.

As a general rule of thumb, for most conventional spring operated PSVs, the pressure drop should be < 10% of the set pressure.

The effects of back pressure is highly dependent on the type of PSV. Some types of PSVs allow for much higher back pressures. Always check with the vendor of the valve.

In Conclusion.

Back pressure consists of two elements: constant superimposed back pressure and built-up back pressure. For most PSVs, the built-up back pressure (= pressure drop in the discharge piping) should be < 10 % of the set pressure.

Further reading:

Discharge piping: problems and solutions (coming soon)

How to calculate pressure drop in pipes (coming soon)

Types of PSVs

Conclusion

PSV design boils down to 5 steps:

• Perform a blowdown & relief study to determine the valve capacity
• Determine the set pressure in accordance with the vessel construction code. Account for constant superimposed back pressure.
• Design the inlet piping with minimum pressure drop
• Determine the safe location: disposal system or discharge to atmosphere
• Design discharge piping with minimum pressure drop. Account for disposal systems.