Booster pump for LPG pipeline

[LPGWORLD]

I´m designing an LPG pipeline and I´ve a main dobut that I´ve found in NFPA598, FNPA59, API2510 and other LPG norm.

My doubt is how to consider the NPSH for a booster pump, not the calculus.

The project is a Marine terminal for unload LPG. Ships containing GLP must be unloaded/discharged into storage spheres of 5000 m3 of capacity each of them (10 spheres). The distance from ship to factory where the booster pumps will be installed is of 4400 linear metres with a cota/level increase of 50 m with a 12" pipeline. Up to that point the GLP will be pumped by means of the ships pumps. The main problem is that the storage spheres are located at a height of 170 m above sea level therefore the booster pumps will be responsible for supplying the necessary pressure to overtake discharge loss, the increase in level and the pressure inside the storage spheres (6 barg).

The ships discharging GLP will be all different therefore it is very likely that conditions such as pressure and temperature will vary. The ships pumps will be able to supply 800 m3/h to the plant where are placed the booster pump

It is very likely that the pressure and temperature conditions reached by GLP when getting to the booster pumps area will not be balanced, which means that the GLP will be undercooling the pressure, the moment of discharge is indeed of utmost importance since at the beginning the temperatures will be higher. The aereal part of the gas pipeline (500 m) is insulated and the remaining part is buried therefore will be gradually cooling and the temperature at which the GLP reach the pumps will be lower.

My doubt is if for to know the supply pressure of the ship, i have to increase the pressure drop in the pipeline to the vapor pressure of the LPG (at ambiente temperature for example) to ensure that at the suction of the booster pump I have liquid phase for its proper work.

It is expected that in normal conditions the pump will run in liquid phase (although some gas phase could be dragged) and variable temperature.

The idea is to install two pumps running in parallell so as to reach a caudal of 800 m3/h nominal discharge, this booster pump will supply the head to reach the storage spheres.

 

[Yuze]

For LPG transfer system, the cooler the liquid temperature and the lower pressure drop before the booster pumps, the higher NPSH available for the booster pumps. I don't see any reasons you need to increase the pressure drop through the pipeline.

 

[Montemayor]

LPGWorld:

Are you designing the eventual installation of an actual LPG import terminal? Or are you designing such a project for academic purposes or as a university assignment? Your vocabulary contains common Spanish terms that should be defined for those on the Forum not familiar with them: GLP = Gas Licuado de Petroleo (LPG); “cota” = altitude over sea level; and “caudal” = flow (and not “the posterior of a body” – as the English dictionary defines the word).

As in any LPG or LNG import project, it is to be expected that the transport vessel(s) will have the responsibility to pump their cargo onto land-based storage tanks. These pumps are usually submerged centrifugal pumps located in each cargo tank. Presumably, you have the characteristic information on these pumps (such as performance curves, NPSHr, BEP, dead-head pressure, capacity control, etc.). Can you share this information with the Forum?

Your ships’ pumps will have to fill and pump through a 2.7-mile long pipe line before arriving at the booster pump station. That means you must supply a means to fill and maintain the pipeline with LPG and also a means to recycle any excess LPG back to the ship – or elsewhere. You have to keep the booster pumps primed with LPG and the ability to start and stop the boosters at will – regardless of what is happening 2.7 miles away at the ship(s). By controlling the booster pumps’ suction line filled and primed all the time, you are fixing the NPSH available to these pumps. I don’t see what your problem in understanding this need is. You control the level of NPSHa available to the boosters by controlling the pipeline pressure drop (you select and control the P/L diameter, LPG flow rate, pumping temperature, etc.) and the specifications for the booster pumps. This is all regardless of what the composition of the LPG is and how much it varies – as long as you take that into consideration in your design, calculations, and Data Sheets for the booster pumps.

Unless you are storing LPG under refrigeration (which I doubt), you will have to probably incorporate a high pressure spray inlet into your storage tanks in order to fill the vessels without over-pressuring. This is due to the probability that you are not vapor equalizing your tanks with the ships. You do not mention this need although you seem concerned that you have a hydrostatic head to overcome. The amount of booster discharge head required to get some cooling from the spray injection may be more than the hydrostatic head you have to overcome.