LPG pump 1

[a1987]

Dear Sirs
i'm fresh mechanical engineer and i'm in trouble with an LPG pump..
i have to evaluate a tender of this pump, and i'd like to have some advice about:

1. is there a book of reference for assensing pump in accordance with API 610?
2.
LPG properties:
T= 83 °C
relative density= 0,557 kg/cm^3
viscosity= 0,137 cp
vapor pressure = 14,57 bar abs
head= 400 ft
flowrate= 166 m^3/h

pump suction = 7 bar g
pump discharge = 13,87 bar g
lenght from 1 stage impeller to discharge = 3 m

Operating point of this pump is near of end curve and very distant for Best efficiency point...and is not in accordance with api 610 referring to preferred region and allowable region..

My question are the following:

1. i have extimated the pressure at eye of impeller (8,7 bar g)...how can i evaluate if lpg flashes??
2. in this situation i think pump is to reject, because its operating point is not affidable and subdimentionate, and then could causes problems to electric motor..in your opinion is my mind correct?

What additional controls would you do?

Best regards

ps. operating point is represented in attached file.

 

[TD2K]

I wouldn't select a pump that was that far to the end of the curve, there's simply no up-size.

The process data sheet for the pump should have provided an NPSHA for the design point (166 m3/hr and 400 ft head), the quoted pump has to be less than this value plus any NPSHA margin your company or client recommends/requires. You don't need to try and calculate if the LPG flashes in the pump impeller eye.

If you are getting confused with some of the numbers, the 7 barg suction pressure the pump is specified for is not at the same operating temperature as the provided 14.57 bara vapor pressure.

 

[stanier]

If I were you I would look to the Corken and Blackmer range for these small LPG type pumps rather than an API pump.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[a1987]

Thanks to all..

I have another question for you, in particoular for TD2K...:
- if i would evaluate if lpg flashes, how could i do?

 

[1gibson]

Evaluate pump NPSHR against your NPHSA. If insufficient NPSHA, pump will cavitate and may be damaged. If severe cavitation, or vapor instead of liquid is seen at pump inlet, pump may "run dry" and be damage will be more extensive / more immediate.

 

[Pumpsonly]

a1987,
pls recheck all the figures in your OP.
1) Vapour pressure is higher than pump suction pressure.
The liquid will flash if the static pressure is lower than the vapour pressure.

 

[a1987]

the figure i have posted is only an example to show position of perating point and to underline it's very close to end curve. it's not the real curve 'cause i cannot post performance curve for professional secret...
the values it's real instead..so in this case the lpg could flash cause static pressure is lower than vapour pressure??could i have a diagram reference? for example propane diagram??butane diagram??what of those?
in conclusion i have conjectured the lpg in question is composed to 50% butane and 50% propane..but i know that in winter and in summer percentage is variable..

 

[maddocks]

A couple of comments - like others, I would look at Corken and Blackmer for this service. Almost all LPG trucks that I've come across use Blackmer pumps and they seem to work quite well. As far as an API pump, that may not be the best choice - look at NPSH curves, evaluate speed effects (1800 is preferred), and check your pump HP draw and efficiency up and down the curve at varying ambients. Make sure you don't select the biggest impeller in the casing, and be sure and upsize your suction line for this potentially flashing fluid. You may actually want to consider the use of a VFD on this service depending on what you're delivering to.

 

[stanier]

http://www.corken.com/selection_guides/index.html

http://www.sterling.pl/PC.pdf

http://www.aurexlpg.com.pl/ang/stacje.htm

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[TD2K]

a1987, if you want to estimate if the LPG will flash you'll need to analyse the suction hydraulics of your system to calculate the NPSHA at the inlet flange and then compare that to the NPSHR from the pump vendor.

 

[BigInch]

Why estimate. We know it will flash at one ambient condition or another, most likely in the suction line just before the pump. It's just a matter of when, which is why it is important for LPG pumps to be able to handle the high % vapor.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[poli60]

a1987,
I suggest you to follow Pumpsonly recommendation (star to him) and recheck your data.
In fact at 83°C and 7 barg you have only gas... and, simply, you cannot speak of pumps.
Briefly speaking, when you say LPG, you have a mix of Propane (C3) and Butanes (nC4, iC4).
At 83°C the equilibrium pressures of C3, iC4, nC4 are about 35, 15, 11 bara. (You can find these data on Perry, GPSA etc).
So, to have liquid LPG at 83°C you need a minimum pressure of about 10 barg.
Your vapor pressure figure, ie 14,57 bara, could be realistic. In this case, you need to have a suction pressure higher of 14,57 bara to have liquid... and to start speaking of pumps.
Regards

 

[a1987]

Poli60 Pay attention, please...
i have an lpg at 83°C that is compressed in a tank by nitrogen, and it has a determinated pressure otherwise it could be a gas..
7 bar g is referred to a suction pumping pressure that must be greater or equal than tank pressure otherwise fluid don't be aspired by pump 'cause don't sussist a positive differential pressure that cause motion..
if i'm going wrong in please correct me...

 

[a1987]

i have extimate pressure at the eye of impeller... it could be 9 bar..
suction = 7 bar
eye of impeller= 9 bar
discharge = 13,83 bar

how can i be sure that lpg do not flash??
i wanna analize every possible problem on the pump 'cause i like so much my work and i wanna be sure that i'm not comminting mistakes!!

 

[BigInch]

I may be wrong, but I don't think you are calculating NPSHR correctly.

NPSHR = tank pressure + elevation from liquid level to pump centerline (if direction is down) - vapor pressure - flow losses.

Assuming that your pump elev is equal to or lower than liquid level, and the minimum possible pressure at your pump is tank P - VP - flow losses, so if tank p and elev both = 0, then you are saying that you have 14.7 - 9 + elev = 5.7 bar flow losses? Way too much for an LPG system. To recover that, you need the pump 352 feet below the tank liquid level. The pump needs to be closer to the tank, a whole lot closer.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek