Reducer on discharge side of cent. pump 1

[shahyar]

Hi,
I know it is not common to put reducer on discharge side of a centrifugal pump. But I like to know WHAT will happen if I put it? (centrfugal pump/blower).
The similar question could be "What if we put expander on suction side of a centrifugal pump?"

Thanks

 

[SeanB]

Just curious, what type of pump is a jet water pump?

 

[JJPellin]

A jet water pump is a multistage pump used to provide the cutting water for cutting coke out of the drum. One of ours may be typical. It is 10 stages, barrel style. The impellers are all aligned with a very large balance piston at one end. The pump produces 1000 gpm at about 3600 psi pumping water that is recycled so it tends to be dirty. The pump starts and stops several times per day. It only runs when they need to cut a drum which could be on a cycle of 12 to 24 hours with one jet pump serving 2 to 6 drums. So, in combination it might run twice per day or as much as 6 times per day for 2 to 4 hours each time.

Johnny Pellin

 

[BenThayer]

i have to admit that we routinely put the flat side of ecentric reducers on the bottom at pump inlets if we think we might have a cleanability issue in our pharma plant.

drainability and being able to prove it can be important to the FDA, etc. and if you do it right, it is not an issue for the operation and reliability of the pump (and even if it was, the FDA and Q would not care).

 

[insult2injury]

JJPellin:

A jet pump and a multistage barrel pump are very different. A jet pump refers to an ejector, eductor, etc... The pump you are talking about is just a multistage centrifugal pump with double casing same as a boiler feed pump. In boiler feed applications (and condensate pumps for that matter) where the pumped liquid is at or very near saturation. Ask any pump manufacturer, and they will agree with the advice of Karassik. In fact with barrel pumps the manufacturer's preference would be to have a completely vertical drop from the source tank to the suction nozzle.

I2I

 

[quark]

shahyar,

You can have reducers in the discharge(one size, as a thumb rule) but pump manufacturers suggest not to reduce the suction piping below the given suction flange size. So, an expander(I am comfortable with the term expander, technically and semantically, and the term has been in use) is avoidable in the suction line.

 

[JJPellin]

When I refer to a jet water pump I am referring to the service. It is like saying a Naphtha Reflux pump or a Fuel Oil Product pump. Jet water pumps could be many configurations, but I have only ever seen multi-stage barrel style pumps in this service. I make this an example, because jet water service in a coker will tend to be the largest, highest pressure, highest suction energy pump in a typical refinery. If there was ever going to be a problem associated with a high point vapor trap in the suction line of a pump, this should be the service where it would be most extreme. But I have never seen or heard of a jet water pump that did not have a very large vapor trap in the suction line.

Johnny Pellin

 

[djack77494]

As much as we may have our preferences and our good and bad experiences with doing things one way or another, there is no substitute for engineering judgement. Generalities may apply in the majority of situations, but there are always the exceptions.

Moving on to the original question and adding something to satisfy the engineer's desire to have reasons for our rules, let's think about the discharge of a pump. Exiting the pump is a highly turbulent, undeveloped liquid stream. The pump vendor is striving to offer a product that economically meets the specification, and a larger size discharge nozzle = more cost to him. The added cost does not equate to added value beyond the point where excessive wear or reduced product life is avoided. Once out of the pump, however, the engineer seeks the optimum size pipe, which typically would result in lower velocities and pressure drops. Thus the piping is typically larger than the pump discharge nozzle.

Late katmar, I'd almost say that this is ALWAYS the case, but if I did so, I'd violate the points I was making in my first paragraph. ;-)
Doug

 

[pumpking]

Many subjects being discussed here, but coming back to the original question, have to agree with BigInch, if your putting a reducer on the pump discharge then it is worth looking at the whole system design, as friction losses of a smaller discharge pipework system will be much higher, therefore, it will more than likely be the case that you can install a much smaller pump to do exactly the same job !! It will also make running costs and energy costs much less over a period of time !!

 

[TenPenny]

I have seen reducers on the discharge. All you are doing is adding restriction and therefore more head to overcome, which then will change the performance of the pump. You'd have to analyze the whole system and the pump curve to see the effects.

Step back a minute and think - you're paying for energy to move a liquid at a certain pressure, then by putting in a pipe restriction, you are wasting energy. It's like opening a window in the middle of winter - you're just wasting energy.

There are times when this actually helps, but in general, it's not a logical idea.

 

[Artisi]

It is very posssible that a reducer on the discharge side is valid - take for instance an installation where a large pump operating a slow speed has been selected for a hydraulic consideration ie, very low NPSHa. It is therefore valid for the discharge pipeline from a pure economic perspective would be of a smaller diameter than the pump discharge flange.

Plus there is no law from the pump police to say that the discharge pipeline must be the same size as the pump discharge flange- pipeline diameter is based on a friction head / economic consideration.

As TenPenny has pointed out "There are times when this actually helps, but in general, it's not a logical idea."

 

[Scipio]

Agreed with Artisi, I've seen reducers on discharge twice for valid reasons, once it was a larger progressing cavity pump run at a very low speed for erosion concerns, the second was a slurry service where the pipe diameter was reduced to keep the slurry above it's settling velocity.