Q: We are happy users of your invaluable series of handbooks for instrumentation and process control. There is something we could not find in Vol. 2, Chapter 8.34, about control of pumps. What would be your recommendation for the control of two pairs of reciprocating pumps installed in series (booster/main arrangement pump) used in a crude oil pumping station? As shown in the image below and in the attached PFD you will be able to understand better the addressed control problem.
The pumping station delivers crude oil to the pipe line, and has two pumping stages, booster and main (each one in a redundant configuration—two operative and one cold standby—with a total capacity of 300 KBPD). All pumps (both boosters and mains) are of the positive displacement type. The boosters are 1200 hp and the mains are 3000 hp each. Each pump has its own hydrodynamic speed driver for velocity control whose setpoint is provided by a selector control between the total flow and the discharge pressure for the booster pumps. For the main pumps, the selector is between the total flow, the suction pressure and the discharge pressure, and, therefore, the controller with the lowest demand will set the velocity to the respective pump. Our questions are as follows:
- For a pumping configuration with reciprocating pumps in series, each one with its own speed control, is it possible to maintain an accurate and stable pressure control just using speed control?
- If a booster pump or a main pump shuts down, would this control system be able to respond in time in order to maintain pressure balance on the system?
- Someone suggested to us to include a control pressure valve to recirculate oil at the intermediate manifold between boosting stage and main stage, which will fast-actuate in order to stabilize the pressure if needed. Would this work?
We appreciate your suggestions and critique. There is too much debate here at my company about this point.
Luis Fernando Betancur
A: You are right, Chapter 8.34 in Vol. 2 does not answer all your questins, you also have to read Chapters 7.4 and 8.35 to get the full picture.
Your solution of using variable-speed (VS) positive displacement (PD) pumps, instead of throttling bypass control valves, is the correct one, because the pipeline process is a mostly friction one, and you would be wasting a lot of energy in the form of valve pressure drops. With PD pumps, cavitation also is less of a problem.
As to installing a recirculating control valve back to the supply tank from the intermediate manifold, this is reasonable if you want to mix the tank contents before start-up (to reduce settling and achieve uniform density) or to gradually increase the booster discharge pressure during start-up to the required inlet pressure of the main pumps before they are started, but not for normal operation. The relief valves should take care of overpressures. You need those plus blocks and check valves on each pump. In addition to the pressure safety valve (PSV) opening in the bypass, the booster pumps have to shut down any time the main pump trips, and, similarly, the main pumps have to trip to prevent the development of low suction pressure, when the boosters trip.
I assume you have selected the booster pumps that require a minimum inlet pressure (net positive suction head required, NPSHR) that is lower than the net positive suction head available (NPSHA). To determine this minimum, you determine NPSHA using the minimum possible supply tank level and oil density in combination with the maximum oil density expected.
The purpose of the booster pump is to keep the main pump suction pressure constant by compensating for supply tank level, viscosity, etc. If we understand "the personality"of the pipeline process, the required controls become fairly obvious. As you can see in Part A of Figure 1, the pump curves (in red) of all VS-PD pumps are very steep parallel lines, because flow changes very little, even with large changes in discharge pressure. If you have a pair of pumps in parallel, pump flows are added (Part B), and if you have a pair of pumps in series, pump pressure is added (ΔPs are added in Part C). In your case, you have both series and parallel operations (Part D), so the combined pump curves are as shown by the parallel red lines. For sake of this illustration, I have assumed that all pumps are the same (which obviously is not the case), but it simplifies the explanation.