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Equipment-based Mitigation: Relief Valves and Air Valves

Equipment-based Mitigation: Relief Valves and Air Valves

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An important aspect of waterhammer mitigation is protecting against pressure extremes – either high or low. Two devices are commonly used to do so: relief valves and air valves (sometimes referred to as vacuum breaker valves). This article looks at what these devices are, how they work, and when they might be appropriate for the engineer to use.

 

Relief Valves

 

A relief valve is any valve designed to open at a set pressure, discharging fluid from the system until the pressure falls to an acceptable level. Relief valves are commonly seen as safety devices to keep reactors, boilers, or other pressure vessels from rupturing. However, they can also be used to mitigate waterhammer and high-pressure waves travelling through a system.

 

In the most basic form, a relief valve consists of a plug that closes off a system exit, with the plug held in place by a spring. When the force applied by the spring is exceeded by the force from fluid pressure, the plug is pushed upwards, allowing fluid from the system to exit through the relief valve to a low-pressure area. This system is depicted in Figure 1 below.

Schematic for a basic pressure relief valve

Figure 1: Schematic for a basic pressure relief valve.

 

In a pressure relief valve such as this, the valve can be partially or completely open, depending on the system pressure. They also operate without human or mechanical intervention, giving them the name ‘passive relief valve’.

 

Passive relief valves are effective at reducing high pressure surges in many systems. However, they have two major drawbacks. First, they can be relatively slow to open, reducing their effectiveness against short, high-speed waves. Second, they can leak when the system pressure is near the valve’s set pressure, meaning the operating pressure must be well below the set pressure.

 

Another type of relief valve is a Pilot-Operated Relief Valve (PORV) which helps address those concerns. In this valve (shown below in Figure 2), the plug is held closed by the system fluid. Instead of using a spring to hold the plug in place and control when the valve opens, the pilot pressurizes the dome with the system fluid to keep it closed. When the set pressure is met, the pilot vents the dome to open, rapidly changing the forces on either side of the plug.

Schematic for a Pilot Operated Relief Valve

Figure 2: Schematic for a Pilot Operated Relief Valve.

 

In this pilot-operated system, the valve is able to respond more rapidly to pressure waves in a system, allowing it to more effectively mitigate waterhammer in a system.

 

Looking at an example of a relief valve in action, Figure 3 below shows a system’s response to a valve closure (followed by a pump trip) with and without a passive relief valve. The relief valve had a setpoint of 400 psig (27 barG). Without the relief valve present, the system pressure reached more than double the relief valve setpoint, showing how effective this device can be.

Graph of system pressure following a valve closure with and without a relief valve

Figure 3: Graph of system pressure following a valve closure with and without a relief valve.

 

Air Valves

 

If relief valves are meant to prevent high pressures in a system, air valves are meant to prevent vacuum conditions in a system.

 

Air valves can be used to allow air to both enter and exit the system. In normal operation, it is preferrable to keep pipes liquid full by allowing air to exit the system via the air valve. However, allowing air to enter the system can be beneficial during low-pressure transients.

 

Pipelines commonly see elevation changes along their length, and that elevation change can cause cavitation to be present at high points even if the rest of the system is seeing moderate pressures. In extreme situations, these vacuum conditions can lead to the pipeline collapsing. Air valves can be employed in these situations to allow air at ambient pressures into the system, breaking the vacuum in the system and staving off potential pipe collapse.

 

Air valves work in one of three ways. The most basic types of air valves will only let air in or only let air out. An air valve which only allows air to exit the system will not be useful at waterhammer mitigation, but can be beneficial in other ways. An air valve which only allows air to enter the system might be used when the risk of process fluid escaping the system cannot be tolerated.

 

The majority of air valves will allow air into the system when the system pressure falls below ambient, and allow air to be expelled when air at the valve is pressurized above ambient. These valves may also be referred to as an ‘air and vacuum valve’.

 

A limitation to this type of air valve is that air allowed to enter the system at low pressure can be violently expelled from the system as it returns to high pressures. In fact, a two-stage valve (where air can both enter and exit through the valve, shown below in Figure 4) can even add high-pressure waves to a system. As the last air exits the valve, the liquid boundary slams the valve shut, halting flow along that path. This air valve slam is similar to an abrupt valve closure and often generates high-pressure waves.

Schematic for a two-stage Air and Vacuum Valve

Figure 4: Schematic for a two-stage Air and Vacuum Valve

 

Thus, the third type of air valve operation is a three-stage air and vacuum valve. The third stage in these valves is a device that limits the rate of air outflow at the tail end of the air-expulsion process. Adding this third stage onto the valve is similar to following the 80/20 rule for valves, in that slowing down the last portion of the process eliminates dramatic system changes, preventing waterhammer events.

 

Looking at an example of a three-stage valve in action, Figure 4 below shows a pipeline’s response to a pump trip with and without a three-stage valve. In this image, pipeline conditions are shown 14 seconds after the pump trip when the pressure front has traveled 33,000 of the 38,000 foot pipeline. Air valves located at high points are allowing air into the pipeline, preventing the substantial vacuum conditions present in the original system.

 

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