Know-How Surge relief valves

Task

In the search for suitable methods to mitigate or even prevent pressure surges, there are a few things to consider beforehand. We approach with the questions:

What is a pressure surge and when does a pressure surge occur?

What is a pressure surge?

Pressure surges are dynamic pressure changes. These can occur when the flow velocity of the medium changes abruptly. The kinetic energy of the fluid is converted into a pressure change. This can take the form of a pressure increase or pressure decrease and can run either in the direction or against the flow direction of the system. The pressure change can be so extensive that either the permissible maximum pressure in the system is exceeded or cavitation occurs in the system as a result of pressure reduction.

Possible damage depending on the type of pressure surge

During the operation of pipelines, dynamic pressure changes are generally unavoidable, but must be kept within permissible limits. Impermissibly high pressure surge loads can lead to major damage:

When does a pressure surge occur?

Pressure surges in general

• During start-up and stop of the pump in normal operation
• When the flow suddenly stops, for ex. in the event of a fire or power failure
• When a shut-off valve is suddenly closed

These are all cases where pressure surges can typically occur. And for this purpose there are valves that remedy this problem.

Pressure surges in practice

• During start-up of pumps by acceleration of the liquid mass 
• Liquid column torn off when pumps are switched off 
• Abrupt stop of a liquid in motion by a quick-closing valve
• Sudden change of flow direction by solenoid valves
• Abrupt change of the pipe cross section owing to constrictions and throttles
• Transient pressure fluctuations can also occur in piping systems with a low gradient or slope 

Self-acting control valves against pressure surges

These valves operate independently of external auxiliary power and do not require additional pneumatic or electric control parts. Therefore, they are particularly suitable for pump systems, feed pumps, deep wells and remote locations, that are difficult to maintain, as well as older systems.

Typical self-acting industrial valves for applications with pressure surge problems are the so-called surge relief valves SRV or surge anticipation valves SAV.

In some cases, surge relief valves can be a cost-effective alternative to a surge tank or hydropneumatic reservoir.

Operating mode of control valves for applications with pressure surge problems 

When the set pressure has either been reached or exceeded, the surge relief valves SRV open in order to expel the fluid from the system. Then they close slowly in a controlled way to avoid a pressure surge. After a power failure, that can often cause a fast, abrupt pressure pulse, the SRV might open too slowly. This is why the SRV is normally used for the decay of a gradual pressure increase and the permanent overpressure control at critical points along the pipeline oder in the bypass line at the pumps.

Surge anticipation valves SAV open when the lower set pressure has been reached or drops below that value and a subsequent high pressure increase (upper set pressure) to atmosphere is expected.

The SAV thus first detects the low-pressure wave, which is typical for an emergency shutdown of the pump (or in the event of a power failure). This is why the SAV is usually installed in a bypass on the pump. By pre-opening, the SAV is already open when the returning high pressure wave reaches the SAV. Sensing the overpressure of the high pressure wave is required to induce opening of the SAV. The SAV closes slowly in a controlled manner so as not to generate a pressure surge. The SAV is slightly better suited for abrupt transient pressure changes compared to the SRV because it can respond and open more quickly.

In order to compensate for an emerging negative pressure wave (e.g. cavitation shocks) and to break any vacuum that occurs, a complete solution consisting of bleeding and venting valves and vacuum breakers is required in addition to the SRV and SAV.

Know-How bleeding and venting valves     Know-How vacuum breakers

Selection of valve type and nominal size

In order that your pressure relief valves (SRV + SAV) function properly, they must be designed to fit your application exactly. The nature of the plant is decisive for the design, e.g. depending on how many valves and pumps are in operation, the flow rate will also vary.

As a rule, the pressure relief valves are operated at approx. 25% of the maximum pipeline flow. Special attention must be paid to the flow rate (K_vs value) and the response pressures. 

Oversizing (SRV + SAV), an excessively large K_vs value or an incorrectly selected lower set pressure (SAV) can relieve too much operating medium to the atmosphere in the event of a pressure surge and thus make plant pressure recovery impossible, since the valve remains open until there is no longer any pressure differential across the valve. This loss of operating medium will persist until the pipeline has run dry, or the liquid column will break off and subsequently cause high pressure rises when the liquid columns collide again (macrocavitation).

Before designing the valves, we therefore recommend that a comprehensive surge analysis be carried out by a qualified engineering service provider. If required, we will discuss the analysis with you and the service provider and will be happy to advise on the correct selection of the valve (SRV + SAV).

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Material selection

Mankenberg valves are available in materials such as stainless steel or duplex steel. Elastomers for O-rings and seals are suitably designed for the requirements of the medium and the operating temperature.

Valve seat leakage

However, SRV and SAV are no shut-off devices which guarantee a tight valve closure. In the closed position, they can have a leakage rate corresponding to leakage classes III or V, optionally IV, according to DIN EN 60534-4 and/or ANSI FCI 70-2:

Leakage class III (metal sealing cone) = 0.1 % K_vs value
Leakage class IV (PTFE seal cone) = 0.01 % K_vs value
Leakage class V (soft seal cone) = 1.8 x 10^-5 x Δp x D* [l/h]
*D=seat diameter