what to consider in sizing / picking the right pump? 1

[Engineerworx]

we are interested in replacing an existing pump with a bigger pump. mainly the reason is we want increase the transfer rate by replacing the pump.

can someone help on the things we should be considering in sizing the right pump.

thanks

 

[DaEarl]

This is a very open ended question.

What kind of pump do you have? Have you examined the hydraulics of the system to determine if there are any oppurtunities to debottleneck the suction or discharge piping? Develop a system curve for the existing system and this will be more clear. With this info you can perform a cost benefit analysis for determining whether piping upgrades will be beneficial if there are oppurtunities to debottleneck the system.

If your hydraulics determine that you must upgrade the size of the pump and the pump vendor can not help by upgrading the existing pump, you must obviusly provide the pump vendors with a system curve and work with them to select a pump for your system.

If you are unfamiliar with hydraulic calcs and their signifigance in determining selection of a pump, then you need to study up or ask the assistance of a more experienced engineer.

 

[Engineerworx]

RCKMurphy,

its a centrifugal(3 x 1.5 ", 3" suction nozzle, 1.5" discharge nozzle) pump for a 9 mile transfer line. the suction and discharge side pipe size is 6". the distance from pump center line to bottom of tank is very minimum, less than 5ft on the suction side. height of the liquid is about 30ft.

on the discharge end, the distance from pump centerline to inlet of tank is about 30ft.

I also wonder what is the maximum flow we can have for a given pump

 

[DaEarl]

What rate do you pump now? The suction and discharge piping are the same size? This is strange to me.

 

[DaEarl]

Do you have the pump curve available? If so, link it to you next post.

 

[Engineerworx]

the current pumping rate is around 200 gpm. yes the suction and the discharge piping are the same.

link to pump curve

http://img126.imageshack.us/my.php?image=pumpcurvehp7.jpg

 

[Engineerworx]

sorry the image was very poor quality

 

[JJPellin]

I would have a few suggestions. First, don't size the pump larger than it needs to be. Determine what the next constraint is to increasing flow. In a refinery unit, there could be a hard limit based on permit considerations, heater, exchanger or reactor capacity. It does no good to buy a pump capable of 500 gpm if the pipeline is only rated for 400 gpm.

Balance the need for a good NPSH margin with the need for a lower suction specific speed. In this analysis, be realistic. The NPSH(r) is sometimes based on a group of worse case assumptions: lowest possible tank level, highest possible product viscosity, highest possible vapor pressure. If the convergence of these unlikely extremes is impossible, then don't design for it. If you are overly conservative about the suction head, it may drive you to buy a pump with a higher Nss which will limit the downturn.

Allow for unusual operating modes. In tank services, we size pumps to transfer the product faster. That is what the pumpers want. But, if they have to drain the tank to change to new RVP requirements seasonally, it can be a problem. When you want to set the floating roof on its legs, or re-float it, you have to go very slowly. A large pump might be incapable of running low enough in flow to meet the regulations for re-floating the roof. You might need a spill-back line to allow for these unusual conditions without destroying the pump. Or it might be worth the price to add a smaller heel pump to handle the low flow condition. I personally, believe that the minimum flow value given by the pump company can often be too low. Determine a more conservative minimum flow based on Nss and specific speed.

Ask what the future might hold. We recently bought a pump that was for a fuel oil desulfurizer. Three weeks after it was started up, they got an idea that they could capture a high profit by switching the unit to hot gas oil. The design of the pump (materials, gaskets, etc.) needs to be able to handle the new service.


Johnny Pellin

 

[twobits]

Knowing the NPSHA and the NPSHR coupled with the TDH and your flow rate will get you started at a pump. If you were to have different conditions as posed by changing tank levels thus different NPSHA you can put a VFD to your motor. By decreasing your speed you lower your NPSHR. You will change your TDH and flow rate but this will help determine your pump. The pump curves will have 1150 rpm, 1750 rpm and 3600 rpm curves. You can fine tune the VFD settings on site.

 

[PetroBob]

Not sure if it's too late for this posting... But incase you didn't think of it already, make sure you check design pressure of downstream piping and equipment, before purchasing your new pump. Larger pump (or pump impeller) will give higher pump shutoff pressure. (Incase you're not familiar with this see thread1203-159440 on this forum)

If you don't have a pressure relief valve on downstream side, and if you do have any valves or any way to close off piping downstream of the pump, then your downstream piping must be designed for pump shutoff pressure of the new pump. And if you do have a PSV, then it may need to be changed out to meet new relief case flow rate.

 

[chief]

The capacity of your pump is dependent on the size (Kw/HP) of your motor. If you increase the pump speed by using a pulley and belt arrangement you will also improve your flow, but you may require to change to a higher Kw motor. With a pipe line this length you can expect large losses due to pipe friction.

Offshore Engineering&Design