Piping System Design for Twin Screw Pump

[ikha88]

Dear All,


I'm Ikha, a new comer here. I've been working as process engineer for 2 years. But I have no experience in piping system design for Twin Screw Pump. I hope somebody here can help me.

In an on-shore facility, which we are designing, has an emergency crude oil transfer pump. Its type is Twin Screw Pump. The philosophy operation for this pump is to transfer the crude oil from Crude Oil Storage Tank to another Station connected with 6"-8 km Crude Oil Pipeline. It is operated for intermittent/emergency service, which is only to break the gel formed in the pipeline by its pressure.

My questions are:

1. Now we have designed the piping system for this Twin Screw Pump only based on velocity criteria for reciprocating pump. We refer to reciprocating pump because both are positive displacement pump. API 14e, as I know is not provide any information regarding piping design for Twin Screw Pump. Is anybody know the suction & discharge design (velocity, piping design) required specifically for Twin Screw Pump?

2. The pump capacity is 117 GPM. Beside this emergency transfer pump, for normal operation there are also 2 units of Crude Oil Transfer Pump which are Centrifugal Pump, have capacity of each 117 GPM, and can be operated for single or parallel (2 x 117 GPM) operation. These three pumps (2 Crude Oil Transfer Pumps and Emergency Transfer Pump) have the same suction header (8") and discharge header (6"). And also have same piping system for each pump, which are as follow:
- Suction pipe (from suction header to pump) : 4" with velocity 2.96 fps
- Discharge pipe (from pump to discharge header): 3” with velocity 5.06 fps

After we clarify to vendor of Emergency crude transfer pump. They provide the suction flange and discharge flange connection of 10" for each. Because it is too different with our calculation. We asked the vendor regarding the technical reasons for large size design of suction and discharge flange, we want they clarify the suction and discharge velocity. But they did not give clear reasons. They just said that minimum suction pipe shall be same or larger than the suction header of the pump. And large size pipe required to ensure the volume of liquid is available for the operation of the pump. And they said nothing regarding the calculated velocity using 10". Of course we calculated to check it earlier and the velocity in 10" just 0.48 fps. It's already below the minimum velocity criteria.

The worst part that the vendor suggested us to upgrade the suction header from 8" to 10" because of their pump. And we are totally rejected because it cause additional cost since the suction header length is 350 ft.

Please kindly your advice regarding my two questions.


Thank you for your advice.


Best Regards,
Ikha

 

[LittleInch]

I'll give this a go to your key points, but despite the length there is quite a lot of key info missing and you are asking us to comment on third party comments with about 10% of the infomraiton. However...

1) Twin screw pumps provide for a relatively smooth pulsation less delivery and so there are few special issues for these pumps AFAIK.
Beware of using API 14E as anything other than an initial guide. It is intended for offshore production platforms and many of the limits it suggests are simply good initial ideas.

2) The screw pump is an almost constant volume pump (~10%). Your centrifugals are essentially (~10%) constant pressure pumps. Just because you turn another pump on, doesn't mean you get 2 x the flow(!) the flow in parallel operation will depend on the intersection of the pump curve and system curve and turning on another pump will commonly get you another 40-60% more flow for the same system, but this is highly system dependant and needs to be worked out for your specific system.

The only thing I can suggest wrt the inlet size is that this is related to what is likely to be a very high viscosity of the crude oil at what I suspect is close to the pour point. That is the reason you have screw pumps because centrifugals don't work too well around the 300cP mark (though this varies by pump and manufacturer).

Given you're trying to break a gell tells me that your crude at the screw pump is pretty gloopy and hence would easily need a bigger pipe and flange, but as you've not provided any technical information I can't say.




My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ikha88]

Thank you for replying, LittleInch.

Below are my answers.

1) Twin screw pumps provide for a relatively smooth pulsation less delivery and so there are few special issues for these pumps AFAIK.
Beware of using API 14E as anything other than an initial guide. It is intended for offshore production platforms and many of the limits it suggests are simply good initial ideas.
--> Yes, absolutely true, but I have to know what are the special issues for Twin screw pumps.[highlight #4E9A06][/highlight]

2) The screw pump is an almost constant volume pump (~10%). Your centrifugals are essentially (~10%) constant pressure pumps. Just because you turn another pump on, doesn't mean you get 2 x the flow(!) the flow in parallel operation will depend on the intersection of the pump curve and system curve and turning on another pump will commonly get you another 40-60% more flow for the same system, but this is highly system dependant and needs to be worked out for your specific system.
--> Ok. I have noticed on that. Actually, in parallel operation the total flow will be 87%.

The only thing I can suggest wrt the inlet size is that this is related to what is likely to be a very high viscosity of the crude oil at what I suspect is close to the pour point. That is the reason you have screw pumps because centrifugals don't work too well around the 300cP mark (though this varies by pump and manufacturer).
[highlight #4E9A06]--> No, it's not viscous oil, because the viscosity is just 0.57 cP. But it has high pour point around 29*C. But it will not be a problem since there is pour point depressant injected in the production & test manifold. It can decrease the pour point until 17*C. The emergency crude Transfer Pump is only operated, if one day, there is gel formed in the pipeline (in case of pour point depressant system failure).[highlight ][/highlight][/highlight]

Given you're trying to break a gell tells me that your crude at the screw pump is pretty gloopy and hence would easily need a bigger pipe and flange, but as you've not provided any technical information I can't say.

 

[LittleInch]

1) I don't know of any special impacts of a twin screw pump that don't apply to any other pump

2) good, a lot of people think pumps just come in "blocks" of flow and they don't...

3) I find it very difficult to believe that your viscosity when you get close to the pour point doesn't suddenly go into the hundreds or thousands of cP. Viscosity is not a fixed number but varies with temperature, especially as you approach the pour point. Please look at that again and advise as it could be the root cause of your issue. If the viscosity didn't increase then there would be no pour point issue - you can't have near gel like behaviour (pour point) and a viscosity of 0.57 cP (better than diesel) so I suspect it is 0.57cp at some elevated temperature quite a bit higher than 29 Deg C??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way