Closed Drain Pump Classic Struggle - NPSHa

 This subject should be well known for mechanical rotating engineer, but do not worry if you haven't face it in your working someday you will face this.

Closed drain system is the bottom line that work to skim all left hydrocarbon from the plant to return back to main process line, the word bottom here is almost literal, yes it will be located at the bottom level of an offshore platform. Process engineers will exploit the free energy, which is gravity, let the gravity pull down the liquid flow, with this concept there is no other way than to put closed drain lower than all process system in term of elevation.

All the explanation above, why in the end we still concern about NPSH? we need to back to simplify formula to calculate NPSHa.

NPSHa = hpressure + hstatic(elevation) - head friction - headvaporpressure, all in meter unit.

If you interest to scientific formula, you may check via fluid dynamic book specific for centrifugal pump subject, it basically conversion from pressure unit to meter(distance) unit.

You may notice in the positive side is head that work as pressure, if the liquid comes from pressurized storage such as pressure vessel, it will help to increase the NPSHa.

If you put the liquid high enough relative from where you put your pump at, it also helps the NPSHa.

In the other hand, if you have suction piping routing that have a lot of bend, it will decrease the NPSHa, but this always being overcome by good piping sizing and layout, that in the end resulting almost negligible friction loss relative to vapor pressure.

The most affecting value that decrease down NPSHa in hydrocarbon application is vapor pressure.

I will try to simplify the definition of vapor pressure based on my limited understanding, vapor pressure is pressure that generated by a liquid in vapor phase, so let say if the ambient atmosphere pressure 1bara, and we have liquid with vapor pressure >1bara, this liquid tend to vaporize, just become progressively become gas for the area that directly exposed to atmosphere, if vapor pressure <1bara, it stay in liquid form.

As you may know a gasoline, it tends to vaporize slowly if you put in an open container, so it vapor-pressure is higher than atmospheric pressure. The bigger the surface area, the faster the vaporize process will occur.

So back to closed drain, the closed drain liquid that mixture of hydrocarbon and water have vapor pressure > 1bara. The closed drain drum is located at the lowest process related to main process deck, then you need to add another small deck below the closed drain drum deck just for closed drain pump to help the NPSHa (usually this deck also have open drain pump here), but your elevation will be limited with seawater level, you would not prefer to increase the entire platform elevation just to accommodate NPSHa of Closed Drain(CD) Pump, bitter truth is CD(closed drain) pump not the core for making money in offshore platform.

Accordingly, how to solve this Closed Drain Pump NPSH issue?

Solution of low NPSHa see the formula for idea

1. VS-6 pump, the vertical canned pump act as another storage vessel, that give you more elevation head.
2. Use pump that able to do suction lift or have very low NPSHr, such as slow RPM positive displacement pump, Air Operated Double Diaphragm pump actually able to serve the job if flowrate and pressure allow, another high CAPEX solution (compare to centrifugal pump) is using progressive cavity pump.

If possible, another quite challenging solution is just to make elevation of closed drain pump lower, so we able to avoid use of VS6 pump and able to use OH2 the simple yet deliver pump, but this needs to be done on conceptual stage (FEED Front End Engineering Design), if in offshore platform, a pit lower than the last deck is made, just to provide additional elevation head. In onshore, we may be able to ask civil to build a pit but maybe need additional rainwater system to pump out rainwater from the pit, a small non-API pump. Yes, it seems a lot just to avoid VS6. This solution perhaps unlikely being favored by project since it is increase CAPEX. However, long term OPEX and availability/reliability OH2 vs VS6 may be a trade-off to this increase of CAPEX.

Famous VS6 Type Pump

The background on why I'm not really in favor to VS6, it is due to the pump usually multistage since the 1st stage act as booster to next stage to reduce the NPSHr requirement, VS6 also usually a tight tolerance design so it is prone to solid deposit which probably appear in closed drain application, since it will take all liquid from all process line. The bad news is, in oil-and-gas, well is sometimes bringing sands. The anatomy of VS6 that using canned is make the matter worst, the solid will sit on the bottom of the can and may be able to jam the 1st stage, then we lost CD pump.

VS6 also give quite struggle on material handling, due to long shaft pump, it needs big laydown area and also high headroom, sometimes, you need to have hatch on the deck above it. Another thing is it is not easy to clean the can since it is much lower than the deck without any access provided.

I also have been seen a closed drain drum (vessel) with boot on the horizontal vessel design, and the closed drain pump is mounted directly on the vessel, it can be VS4 pump type it is single stage (which my favorite), but now I'm curious, how the process engineer able to reduce the vapor pressure on this kind of closed drain drum, did they flash(vent) it before going to the CD drum? Interesting.