Positive displacement (PD) pumps do not function well if the discharge is blocked. Unlike centrifugal pumps, they have little tolerance for flow restriction. A piston pump will try to continue stroking regardless of whether its discharge valve is open or closed. This means that something will have to give—a tripped motor, ruptured casing or worse.
Centrifugal pumps also do not operate well with a valve, but they can at least tolerate a brief closure. In fact, some require a closed valve pressure reading during performance tests at the factory. This difference is the result of the design of both pumps. No internal motion occurs to displace the fluid inside a centrifugal pump. The impeller simply keeps spinning the fluid if the valve is closed. In a PD pump—such as a piston pump—the displacing motion acts on the fluid, and the fluid must exit the pump.
For these reasons, a PD pump needs to have a relief valve (either internal or external), which could be set to open when a certain pressure is reached. This is necessary to protect the pump. Unfortunately, because of pressure pulsations inside a PD pump, knowing at which pressure to set the release valve to open is difficult. Should it be average, maximum or minimum pressure? An example of such a dilemma—raised during a recent conversation with a colleague in the industry regarding release valve pressure settings—is shown in the next section.
Release Valve Pressure Settings
From an email sent to Lev Nelik: I am responsible for determining the adequacy of existing relief valves on the discharge of some of our PD pumps. The original design basis of the release valve sizing was to match the design flow rate of the pump. The engineering and construction contractors I am currently working with say that this design is deficient and should be larger by a factor of π/2 (π = 3.14) to account for the rotary action of the driver acting on the piston in this pump. I cannot find references to this requirement in the open literature and wanted to ask your opinion.
Tom Morrison, BP
Lev Nelik responds:
A release valve opening should be capable of passing the full flow at the set pressure of the valve. For example, if the pump has been moving 1,000 gallons per minute (gpm) at 100 psi and the relief valve was set to protect/bypass at 120 psi, then when it opens, the entire 1,000 gpm should flow through it.
The changeover from main flow to bypass will not be instantaneous because it cannot be in practice. For example, at 120 psi, the valve may just begin to open, and full flow is reached by 123 psi, but not at 150 psi. Different designs of release valves can narrow or widen this “band,” but in general, it should be a relatively sharp change from main discharge flow to bypass.
|Figure 1. System curves
Tom Morrison responds:
Thank you for the prompt reply and information. From your response, it looks like pump vendors would design the release valve for the normal, full-flow design condition. There is a specific nuance to the example you mentioned that I want to make sure that I clarify and understand.