Positive Displacement Pumps in Series 5

[PhilOos]

Good day,

I need some insight into positive displacement pump theory. I realise the differences between a normal centrifugal pump and a positive displacement pump for example, but I want to know how they perform in a system e.g. in parallel or in series. Nowhere can I find literature which explicitly states that positive displacement pumps does not perform or do perform the same as centrifugal pumps in a parallel or series arrangement.

Any help (proof or references) will be much appreciated.

Kind regards

Philip Oosthuizen
Company info:
SteinMuller Engineering Services

http://www.steinmuller.co.za/

 

[BigInch]

Right Katmar, the only way to control is by recirculating some of the discharge to suction. Pressure in the line to the burner is usually not specifically controlled anywhere, being determined by the head loss needed to feed the burners at the required flowrate, that flowrate set via PD speed control. The lower pressure is surely due to the lesser head loss from the lower viscosity oil. Less backpressure would be the result of loading the fuel line with lower viscosity oils. I'd guess there is absolutely nothing wrong with anything.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[PhilOos]

Hi all,
I've looked at two ways to couple two PD pumps in series.
(a) two pumps installed directly after each other.
(b) two pumps installed about 75 meters apart. (see picture).

For a given restriction (causing a back pressure of 38 bar).
For configuration (a), both pumps share the differential pressure, each operating with a differential pressure of 19 bar.
But for configuration (b), the first PD pumps act as a booster pump for the second pumps.

I've ran a couple of tests on site and I do see a notable increase in supply pressure. But I find it difficult to proof theoretically. I still think it has something to do with the spill flow rate within the HP pumps that sees a lower differential pressure?

Thanks for all your replies guys! I learn a lot.

Philip Oosthuizen
Company info:
SteinMuller Engineering Services

http://www.steinmuller.co.za/

 

[BigInch]

What's the length and inside diameter of the fuel line from 2nd pump to burner tip and the viscosities of the two oils?

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[PhilOos]

Hi BigInch,

The reticulation system consists mostly of 4" pipes and the overall length is about 900 ft. I use the same oil throughout the system (Bunker 150), it is only the temperature of the oil that varies. For the low pressure side, the oil temperature is about 83ºC (182 ºF), Oil viscosity is about 15 cSt. For the high pressure side, the oil temperature ios currently 115ºC (239 ºF) with a viscosity of 7.9 cSt. The current HP temperature is too high...hence the pumps struggle to supply oil at the required pressure.

Kind regards,



Philip Oosthuizen
Company info:
SteinMuller Engineering Services

http://www.steinmuller.co.za/

 

[BigInch]

I just ran 900 ft of 4" diameter pipe with a flowrate of 800 gpm at both 7 cSt and 15 cSt.

Changing from 15 cSt to 7 cSt would result in a drop of differential pressure across that 900 ft pipe of approximately 3.5 bars.

I don't see what flow rate you have. Is it near 800 gpm?

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[hydtools]

I think your efforts are misdirected by monitoring pressure with varying viscosity. You should be monitoring flowrate.

You are blaming the pump for the varying pressure as the viscosity changes when it is the system load(pressure) that changes with viscosity changes.

Ted

 

[BigInch]

These viscosities are quite low to expect to see significant changes in the pump's performance.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[PhilOos]

Hi BigInch,

The flowrate I'm looking at is about 840 liters/min (+-30 ft3/min).

Thanks for all the replies, I've gained a lot of insight into the problem.

Kind regards

Philip Oosthuizen
Company info:
SteinMuller Engineering Services

http://www.steinmuller.co.za/

 

[katmar]

Phil, it is still not at all clear what your real problem is - and until we know what the problem is, it is impossible to give solutions.

1. Is the 840 liter/min flow the desired or the actual flow? Whichever one it is please also give the other, i.e. what is your desired flow and what is your actual flow.

2. You have said that you want a delivery pressure of 38 bar, but you are achieving only 34,5 bar. What is bad about having only 34,5 bar? Why does it matter? What advantage would you get if you had 38 bar?

There are 27 replies in this thread, without getting any closer to an answer. We will continue to go around in circles until we know what the real problem is.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[PhilOos]

Hi katmar,

The actual question that I've asked was:
Why do two PD pumps in series (coupled directly after each other) have a different characteristic than two systems in series (each system with its own PD pump)? Reading through all the replies I've managed to figure it out...

Regarding the pressure requirement: The burner nozzles installed on site have a pressure atomization mechanism to spray the oil. This mechanism requires a maximum pressure difference of 37 bar. Taking atmospheric pressure into account leads to an absolute pressure requirement of 38 bar.

I've noted that the actual PD pump screws are erroded quite a bit. I've also noted a thick deposit layer inside all the piping. The deposit has the consistancy of grinding paste and does not change viscosity with temperature. I've asked to get the deposity tested in a lab (still await the results).

Do anyone know how to filter heavy fuel oil effectively..but I guess this question will be for another forum or thread?

Kind regards

Philip Oosthuizen
Company info:
SteinMuller Engineering Services

http://www.steinmuller.co.za/