One of the challenges in relief valve sizing is ensuring the stability of the relief valve. Though relief valve sizing and hydraulics are generally performed assuming “steady state” flow, in reality, the flow is often dynamic in nature. The flow rate through the valve may be affected by the changes in the inlet and outlet pressures. This presents difficulty in predicting how a relief valve will act during a given relief event.
There are three main types of dynamic responses to variable flow conditions:
- Cycling-the opening and closing of a relief device at a relatively low frequency.
- Occurs most often when the relief rate is small compared to the capacity of the relief valve;
- Usually does not cause detrimental damage to the valve;
- May affect reseating of the valve and cause wear over time.
- Fluttering-the rapid reciprocating motion of the moveable parts of the relief valve due to system dynamics.
- Disk does not contact the relief valve seat;
- May lead to rapid wearing
- Higher probability of the relief
- Chattering-the relief valve opens and closes at a high frequency
- Possibility of valve seat damage and/or mechanical
- May result in loss of containment
- More severe during liquid relief
There are a number of factors that may contribute to relief valve instability. One of the most common is excessive inlet pressure loss. With excessive inlet pressure loss, the relief valve will open at set pressure then quickly reclose as the pressure at the inlet is reduced due to the inlet pressure loss. As this cycle of opening and closing continues, instability occurs. In general, the total nonrecoverable inlet pressure loss should not exceed 3% of the set pressure of the relief valve. This becomes more difficult at low set pressures and especially for pressure vacuum safety valves (PVSVs). Certain relief valve orifice area and inlet/outlet size combinations may have difficulty achieving a 3% inlet pressure drop.
Even when the inlet pressure loss is limited to 3%, this may not guarantee relief valve stability. This is a complex issue and the effect of interaction between factors may affect instability. This phenomenon continues to be researched and studied.
For a more detailed discussion of relief valve stability, see API Standard 520 Part II, Section 7 or contact us for assitance.