Sizing Pumping system for LPG transfer

Sizing Pumping system for LPG transfer

On 25 January 2012 15:32, farid farid wrote:

Dear SLA
Good day. I need your assistant to check my steps to size a LPG (liquefied gas petroleum) pump as I know u have a very good experience in pump include for LPG service. The intent of the pump is to transfer the LPG from bullet mounded LPG tank to the other place. I’ll tell you the steps in order for me to size a pump and appreciate for you to correct me if got mistake.
1. First step is I need to determine the flow rate. Generally, the pump flowrate for LPG should be around 30-50m3/hr. but this flow rate is depends on the client. Let say if the client want to transfer the liquid 70m3/hr, I should size the pump based on that flow rate right?
2. Determine the static head. For the open system for me no problem in order to determine the static head. For the pressurized bullet tank, I am not so sure since the bullet tank is under pressure and for me of course it will affect the static head. Can you help me on this since I am a little bit confuse on this

3. Determine the friction head
The friction head depends on the flow rate, the pipe size and the pipe length and fittings along the pipe. If I not mistaken, for LPG systems, the pump should be as close as possible to bullet tank in order to minimize the friction head. It is because we deal with the LPG which is high vapor pressure (around 10 bar) and easily vaporize.
4. Calculate the total head
Sum of the static head and the friction head both in suction and discharge pipe
5. Select the pump
Select the pump based on the pump manufacturer’s catalogue information using the total head and flow required as from the calculation above

I hope you can correct me if got any mistakes and add some more suggestion if you have.

SLA’s response on 26th January, 2012 -

Dear Farid,

You have chalked out the sequence quite good !

The crucial point is the time required to unload the bullet-tanker lorry. If you unload a 70 m3 tanker at 35 m3/h, it will take 2 hours to unload. Is this too long a time ?

By the way, when you mention "...the client want to transfer the liquid 70m3/hr, ...", the client does not know the flow-rate as 70 m3/hr. He knows the volumetric capacity of the tanker, say 70 m3. He would also know the time that unloading operation should take. If 70 m3 have to be unloaded in 1 hour, then pump's flow-rate should be more than 70 m3/hr. Note, 70 m3/hr and 70 m3 are two different things. 70 m3/hr is flow-rate and 70 m3 is volume.

The pressure in the bullet and pressure at discharge of pump, both will have to be higher than the vapour pressure of LPG. If LPG is to be unloaded into a pressurized container, the differential pressure needs to be converted into equivalent metres of liquid column (mLC). These should be added into static head. Such addition becomes necessary if one wants to draw the system resistance curve.

It is always safer and necessary to ensure that the pump is as near to the bullet. Important thought is that there should be no chance for pressure of LPG to drop close enough to its vapor pressure.

I hope I have answered your points.