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Because all hydraulic and pneumatic systems should contain a relief, they can undergo blowdown. Blowdown typically results from friction or other energy losses due to change in a process operation’s sense of direction (reversal). Its mechanical analog is deadband or backlash, and an electrical equivalent resembles hysteresis.
Imagine a simple ball type relief valve as shown in Figure 1. The system pressure to be controlled is p1 acting on A1. When the ball lifts at the set pressure, fluid is dumped downstream to tank at a certain backpressure p0. The term backpressure only refers to the downstream pressure at the outlet of the valve.
For the valve shown in Figure 1, fluid flow occurs across the seat area A1 (an orifice) and is driven by the pressure difference between p1 and p0. Normally, this backpressure eventually leads to a sump at atmospheric pressure, such as a reservoir. Sometimes, however, the diameter of downstream tube, pipe, or hose is not large enough to handle the flow, so the backpressure applied to the valve seat rises.
Alternatively, if substantial static pressure exists in the reservoir fluid at the end of the return line (as in a pressurized reservoir or one that is extremely deep), p0 will exceed atmospheric pressure. This can cause excessive backpressure, thereby reducing return flow. The higher static downstream pressure changes the force balance in the valve, affecting the set point of the valve.
Blowdown, however, refers only to the system pressure, p1. When the system pressure rises to exceed the set point, the valve opens, and flow commences. However, the valve will not reseat when system pressure drops back down to the set point. Instead, system pressure continues to fall. The valve will close when conditions are correct, but at a lower value than the opening (set) pressure. The difference in pressure between the set point and the final closing point is called the blowdown pressure drop, and it is usually specified as a percent of the set pressure.
A certain amount of blowdown is desirable to prevent limit cycle oscillations. Excessive blowdown, though, usually leads to reduced system performance. Within fluid systems, two effects primarily cause blowdown:
- Geometry change in the relief valve as it opens and closes, presenting a variation in pressure-sensitive valve areas.
- Redistribution of internal pressures on the working, sensitive (operational) valve areas. This often occurs when flow within the valve relieves the overpressure during the open part of the relief process.
Nonetheless, a certain amount of blowdown is desirable as a stabilizing mechanism. Suppose a valve is set for 1,000 psi in a system with rising pressure. When the pressure rises to 1,000 psi, the valve opens and triggers relief flow. It’s possibly due to system mass and inertia that some system overpressure occurs before the flow begins to relieve the system. The poppet remains in the open position and, subsequently, the relief flow begins to correct the pressure by dropping it.
In this case, we assume a “perfect” valve that does not exhibit blowdown. When system pressure drops to the set point of 1,000 psi, the valve slams shut. The entire leg of moving fluid suddenly stops and the kinetic energy transforms to a pressure rise. Water hammer ensues, and the valve encounters more than the 1,000-psi set pressure. This causes the poppet to lift, which allows relief flow to proceed again. The flow relieves the pressure and the valve poppet shuts again, producing another cycle of water hammer. This process can repeat, causing limit-cycle chatter. Ultimately, such a “hammering cycle” can destroy the valves. If mechanical resonance occurs within the valve, it may couple with the limit cycle and cause fatal results. Some relief valves are designed with fluid-damping chambers to dissipate energy and prevent further harm.
A different scenario arises with a “less-than-perfect” valve that exhibits blowdown. For example, the valve opens at 1,000 psi, and after a short time, the pressure begins dropping below 1,000 psi into the blowdown zone. At 800 psi, the valve finally slams shut, stopping the relief flow. This transforms the kinetic energy to a momentary pressure rise, say 950 psi. The 1,000-psi valve set point, however, keeps the valve shut. In this case, blowdown was a benefit because it prevented limit-cycle chatter.