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Suction & Discharge Pipe Size

04/04/2013 5:03 AM

There is one case where the line sizes specified in P&IDs are lower than the offered models by all pump vendors.

The suction & discharge line sizes are 2" and 1.5" respectively whereas the offered pump models are having 3" and 2" respectively.

The pump rated flow is 5m3/hr and Rated Total Diff. Head is 235m (low flow high head).

Kindly let me know if this pump can operate trouble free by connecting to the lower sized piping using adaptors?

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#1

Re: Suction & Discharge Pipe Size

04/04/2013 5:29 AM

The 2" and 1.5" pipe sizes are reasonable for the specified flow. The pump may possibly have larger connections to accommodate other impeller/speed choices (or perhaps to accommodate passage of large solids).

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#2

Re: Suction & Discharge Pipe Size

04/04/2013 5:53 AM

...although a quick phone call to the pump vendors would be worthwhile, just in case.

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#3

Re: Suction & Discharge Pipe Size

04/04/2013 10:28 AM

And make sure that the adapters are properly configured and sufficiently down or upstream to provide laminar flow into the suction to prevent turbulence and upstream enough so as not to affect the performance due to backing up the fluid into the discharge nozzle.

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#4
In reply to #3

Re: Suction & Discharge Pipe Size

04/04/2013 10:35 AM

<...laminar flow into the suction...> The Reynolds Number will be far too high unless the fluid were something like strawberry jam or golden syrup.

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#5
In reply to #4

Re: Suction & Discharge Pipe Size

04/04/2013 11:39 AM

I'm using the term laminar meaning as opposed to being turbulent.The following was copied verbatim from a paper by Flint Evans of Lang Engineering Equipment:

"Rule #2. REDUCE THE FRICTION LOSSES

When a pump is taking its suction from a tank, it should be located as close to the tank as possible. This reduces friction losses on the NPSH Available. However, the pump must be far enough away that proper piping can supplied to the pump. Proper piping means that a straight shot of pipe is supplied to the pump that is at least ten (10) diameters of the pipe. We can this the 10D Rule. For example a minimum of 20" of straight pipe must be immediately in front of the pump if the inlet pipe is 2" in diameter. Pipe friction is reduced by using a larger diameter pipe. This limits the linear velocity, hence the friction losses. Many industries use 5 to 7 feet/sec., but this is not always possible.

Rule #3. NO ELBOWS ON THE SUCTION INLET

It is never acceptable to install an elbow on a suction flange! There is always an uneven flow in an elbow. When it is installed at the suction inlet of the pump, it introduces an uneven flow into the eye of the impeller. This can introduce turbulence and air entrainment, which may result in impeller damage and vibration. The only thing worse than an elbow on inlet of a pump is two elbows. As mentioned above, the established method of ensuring a laminar flow to the inlet of the pump is using the 10D rule, straight pipe into the pump. This also means no valves, reducers, tees, etc."

I would tend to agree with most of what he wrote. My job was selecting the correct equipment and later figuring out who screwed what up so that it wasn't performing as expected. Piping configuration was sometimes involved. The original posting never described the fluid and I fail to see any reference to it being a highly viscous material.

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#6
In reply to #5

Re: Suction & Discharge Pipe Size

04/04/2013 3:36 PM

Turbulence begins at Reynolds Number over 2000 to 2300. If the pump were pushing water, a quick back-of-the-envelope calculation gives the Reynolds Number as around 15000. So assuming similar density the flow cannot be laminar unless the viscosity goes up by a factor of more than 7 on water. As the forum still doesn't know the fluid, strawberry jam and golden syrup were given as examples - sweeteners, perhaps.

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#7

Re: Suction & Discharge Pipe Size

04/04/2013 4:31 PM

It is never a good idea to restrict the flow INTO a centrifugal pump. The discharge may be restricted as long as there is ample horsepower and available head to satisfy the head exerted by the discharge system. Two inch piping into a 3" pump may be possible, though not recommended. It will depend on the pump characteristic curve and, as others have said, the viscosity of the liquid being pumped. The discharge head specified is rather high for this size pump so the added restriction of a reduced size discharge may not allow the pump to deliver the required flow. As was suggested above, check with the pump vendor.

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#8

Re: Suction & Discharge Pipe Size

04/04/2013 10:57 PM

Perhaps I am a bit late here and Mr. Vishal has already sloved his problem. Anway, for what its worth, let me give some input :

1. Many comments regarding type of medium being pumped is the key to Vishal's question.

2. Flow Rate (kg/hr) = Area of pipe cs(m2) x Flow Velocity (m/sec) x 3600/ Sp.Vol of medium (m3/kg)

3. To ensure no cavitation at pump suction , Suction velocity < 3 m/sec.(Max) Normally we use a Suction velocity ranging from 2 - 2.5 m/sec. Using this data and the flow, Q, we get the optimium suction pipe diameter.

4. Vishal needs to determine the Specific Volume of his medium and put it into the above formula to work out his optimum suction velocity.

5. Regarding not having any Valves and reducers at the suction line is the right thinking, but may not be the most practical solution since maintenance requirements may necessitate pump isolation during operation.

I would tend to agree on the 10D concept, but always follow the rule that the Suction pipe diameter to be one (1) size larger than the suction nozzle of the pump --> this means that for your 3" pump suction nozzle, the suction pipe diameter should be 4".

Having said this, it means that the 4" suction pipe needs to be reduced to 3" at the pump suction. Here we should use an eccentric reducer 4" x 3" with the flat end on top to prevent any air pockets. Have this reducer as close to the pump suction. By calculation, you will find that the suction velocity for the 4" pipe diameter will satisfy the 2 - 2.5 m/sec suction velocity guideline.

Cheers!

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#9

Re: Suction & Discharge Pipe Size

04/04/2013 11:58 PM

Dear Mr. VishalB,

Your Question is clear, but you have not mentioned - the pump application, whether it pumps water or some other fluid.

If it is water, Pl. take the Velocity as - 1.0 to 1.25 M/Sec., for SUCTION, and 2.0 M/Sec. for Delivery side, and you have the discharge and calculate and adopt the nearest higher size.

If the fluid is slightly viscous, then Velocity is to be less. Let us see other Members views.

DHAYANANDHAN.S

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#10
In reply to #9

Re: Suction & Discharge Pipe Size

04/05/2013 12:07 AM

The lower the Suction Velocity permitted, the larger would be the suction pipe diameter. It is better for the pump, of course, but it is all a question of cost-benefit. I have used a suction velocity of 2.5 m/sec average and 3.0 m/sec maximum quite successfully.

For discharge velocities, it can be higher - upto 6 -7 m/sec.

Satish.

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#13
In reply to #10

Re: Suction & Discharge Pipe Size

04/05/2013 5:00 AM

Dear Mr. SatishMenon,

You are right that cost-benifit comes into picture. It is advisable to have ONLY laminar flow and for lesser velocity even though cost goes up, in the long run, it pays back.

Second point is that the fluid should enter the pump with a very low velocity while coming in to contact with the revolving impeller. With High velocity, the DYNAMICS changes.

DHAYANANDHAN.S

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#15
In reply to #13

Re: Suction & Discharge Pipe Size

04/05/2013 9:47 PM

Dear Mr. Dhayanandhan,

Agree with your input that suction flow should be laminar as far as possible:

Laminar Flow Re < 2100

Transition region : 2100< Re < 4000

Turbulent Flow : Re > 4000

The problem could be that Mr. Vishal may be operating at the Transition region- an in-between zone. To my opinion, the calculation of the Suction velocity is the the limiting factor. Since it is condensate water and he's getting a Suction velocity of 1m/sec, that is wonderful. Mr. Vishal, I don't think you need to worry about the suction pipe sizing. What I mentioned is the best practice.

Fully agree that any Turbulence in the flow could seriously affect the pump impellers - the first to show symptoms of cavitation would be at the suction casing of the pump, and in cases of multi-stage pumps, the suction impeller ( the largest diameter impeller) would be a victim. Pitting can be seen with pockets of metal gauged out of the impeller casting, leading to gradual failure. Unfortunately, if not detected soon enough, this failure may also affect the other parts of the pump like the shaft, bearing, etc. Thank you for this input, since I think that Mr. Vishal would appreciate this info of operating a pump under turbulent flow.

Satish Menon.

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#11

Re: Suction & Discharge Pipe Size

04/05/2013 2:14 AM

Thanks all of you for the responses.

I am sorry to have missed the fluid which in this case is process condensate (water with dissolved gases) with viscosity 0.784 cP and S.G. 0.996 at pumping temp. 31 deg C.

As suggested I checked the velocity at suction for 2" dia size. It comes around 1m/s, well within the limits indicated.

Do we still need suction pipe size higher than the nozzle size? If that is the case our P&ID may need a revision which may further depend on vessel nozzle size from where the piping is emerging.

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#12
In reply to #11

Re: Suction & Discharge Pipe Size

04/05/2013 3:25 AM

Well, what did those pump suppliers have to say in response to the telephone calls? Sheesh!

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#14

Re: Suction & Discharge Pipe Size

04/05/2013 7:52 AM

One vendor is OK with the adaptors and the other needs change in piping size itself as they have chosen the lowest size model already for the specified duty.

We still have to see whether it is possible to change the suction line size coming from nozzle of an existing vessel, for which a revamp is not possible at the moment.

If fluid velocity is within limit, what problem the pump may face due to smaller suction size?

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Users who posted comments:

dhayanandhan (2); PWSlack (4); Satish Menon (3); Spinco (2); The Commoner (1); Tornado (1); VishalB (2)

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