Clarification on the acceleartion head loss in recip pump

[mucour]

Hello,

Thank you for the explanations given in the thread: thread124-196619.

However, Doug and BigInch should clarify a conflicting statement concerning that the heat exhanger cannot be regarded as doing the work of a suction stabilizer. According to BigInch, the heat exchanger breaks the velocity variations but Doug did not feel so.

I am confused to say the least, because I am reviewing a design where a long length of pipe of about 50m is used to connect a tank to 4 pumps, which are in parallel arrangement. In between the parallel pumps and the 50m pipe length from the tank is a large sized suction header/manifold of 16". The 16" suction header feeds the 4 pump suction, which are 8" diameter short length lines. According to the designer, the acceleration head loss is calculated without including the 50m length in the API 674 formula because it is assumed that the 16" main suction header/manifold that is before the parallel pumps will act as a reservoir and it will neutralize the velocity change variation caused by the action of the plunger.

Can someone confirm this assertion, because a pulsation dampener or suction stabilizer has a compressible inert gas (e.g. nitrogen) in the bottle that is used for smoothening the pumped liquid during the reciprocating action, whereas the 16" suction header does not have this compressible gas.

So is it correct not to include the 50m length 16" pipe in the acceleration head loss calculation?

Thanks.

 

[BigInch]

Include the extra length to be conservative.

A gas actived dampener would act quite differently than a heat exchanger. Since that conversation began with dampners and wound up talking about a heat exchanger, I might not have fully considered the transition of concept between the two devices. In any case, I see it as
basically a question of compressibility of the fluid, possibly its vapor pressure and the total volume contained in all parts of the suction line. As such, any volume reservoir would help, but how much would be subject to the exact conditions.

A volume reservoir would act to reduce pulsations; very much so with gas action, however the effect from such a volume reservoir on a typical liquid would be orders of magnitude less. If operating near a fluid's vapor pressure where negative pulsations tended to reach vapor pressure and column separation occured, a heat exchanger might be effective in stopping pulsations from traveling further in the pipeline, however at the cost of large surge pressures in the HX. Perhaps a transient analysis of the suction line would be of some benefit, but I would include the extra length of piping in a hand analysis first to see if further considerations were warranted.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mucour]

Thanks BigInch.

But on doing further research, I came across the concept of applying a kind of “stand-pipe” that is located upstream a PD pump. In a pump article, the introduction of a stand-pipe was used to reduce the acceleration head loss in a situation when the suction length of the piping to the PD pump is considerably long. The stand-pipe is positioned close to the PD pump. The top of the stand-pipe is routed back to the tank for venting the gases.

The only clause is that the stand-pipe should be higher than the liquid level in the tank and the stand-pipe volume should be more than 15 x (times) the pump liquid displacement.

I see a suction manifold placed between the pumps and tank as away to reduce acceleration head loss, similar to the stand pipe.

So if the heat exchanger will provide enough head, then it can also help to reduce acceleration head; as a result only the laterals (i.e. individual pump suction length) can be considered in the acceleration head loss calculation. So far as the vessel liquid volume is more than sufficient for the pump requirements.

 

[BigInch]

The stand pipe is acting the same as any gas activated dampner would, if that gas was at atmospheric pressure (or tank pressure in the case you mention). Its just using the Earth's atmosphere as the pressure source. Generally its not going to be very convenient to have a stand pipe 22 feet or even higher sitting around in front of the pump suction. But remember these things are using the compressibility of gas over the liquid as a pulsation buffer. A heat exchanger pipe flowing full would not tend to have gas or trapped vapor inside, thus liquid being relatively incompressible, would not tend to dampen very much.

Note, that if you didn't have a NPSHa equal to that required to avoid these pulsation problems, installing a heat exchanger, or a header, at any elevation in the suction piping would not increase head, as any head gained by counting the downcomer drop into the final suction line going into the pump would be lost in the riser into the HX. You would've simply created a siphon through which any pulsations could travel.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/