Throttling Suction Valve 2

[arm83]

Guys, I got this quote from a website:

"Pumps are normally throttled with a discharge valve, but in rare cases it can be done with a suction valve.

1. You must have sufficient NPSH to prevent cavitation.
2. Suction throttling prevents the over heating caused by discharge regulation. This can be important with fluids like jet fuel where the additional heat could vaporize the fluid. "

Is the above correct information? I have read in most books that the suction valve should ALWAYS be fully open in order to prevent cavitation and uneven forces in the pump. Can someone please clarify?

 

[rzrbk]

The cases this could be applied would be rare, and I am not sure if there would be any benefit. A discharge throttling valve would be a more reliable set-up.

*The amount of excess suction pressure would need to equal a significant chunk of the pump's differential head to allow control.
*Pump would still be at the same flow-point on its curve so there is no benefit regarding discharge recirculation. If this is a concern, resizing the pump or installing a min-flow bypass is a better option.

 

[BigInch]

Suction throttling to prevent overheating is rediculous, for two reasons,

1.) Overheating usually means you're trying to operate too long at a flowrate lower than 20% without recycling. A ludicrous thing to be doing, if you're not starting up on the way to at least 60% flow.
2.) And the reasoning is garbage. What value is trading not overheating for cavitation? Cavitation will cause more damage, simply because its not so easy to detect. There are usually shut downs for hi temp, but there arn't shut-down alarms for "cavitation", so by the time you notice it, the damage has already been done.

In most cases you should always be in the area of 50-100% of BEP, or it indicates you have other problems having to do with system design and operation, or perhaps poor pump selection (well... that's the same thing isn't it).


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[786392]

Generally speaking BigInch 'as usual' is 100% correct for most of the pump(s)employed e.g Centrifugal,Rotary(Gear, Lobe, Cane etc.)& most reciprocating types.

Occasionally in Certain operating circumstance of typical suction pipeline(s) scenario,Pump's immediate suction Gate Valve throttling is employed by some operation foremen with a concept to block/restrict air/vapours inflow from partially filled flowing suction pipeline?

I do not advocate or second this approach; but this has been field observed.

Best Regards
Qalander(Chem)

 

[BigInch]

You might have to do a lot of things while starting, but once its running, suction throttling isn't the smartest thing you might do. Actually, if you have the luxury of too much NPSHA, I suppose there's nothing really wrong with it, except developing a bad habit. How many times do you have too much NPSHA, and if you suction throttle, you'll probably run out of it sooner or later anyway.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Agreed, cannot see any sense at any time to throttle the inlet side of the pump, even to throttle the discharge side is poor practice unless there is an extreme short term requirement.

 

[MikeHalloran]

One time I had a car equipped with a suction throttling valve in the air conditioning compressor inlet, to modulate the capacity. I.e., the compressor (two cylinder recip) ran continuously, but this temperature controlled valve restricted the flow of refrigerant gas to the inlet.

It worked okay for a couple of years. When the valve died, it failed open, so I rigged a temperature sensor, and converted it to clutch cycling.

( Sensing the expansion orifice temperature worked as well as sensing pressure, but needed a 0.1 Hz low pass filter to keep it from short cycling the clutch. )



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Oh. I almost forgot. The STV was all the rage while it was in production, because you couldn't feel the clutch cycling (because it wasn't), but some of them must have failed within warranty. They went out of production in a year or two.



Mike Halloran
Pembroke Pines, FL, USA

 

[786392]

Nothing from OP as yet, are discussing in air?
or going in wrong direction?

Best Regards
Qalander(Chem)

 

[25362]

FYI, this was taken from the Pump Handbook by Karassik et al. (McGraw-Hill) on the subject of suction throttling.

...Jet engine fuel pumps frequently are suction throttled because discharge throttling may cause overheating and vaporization of the liquid. At very low capacity, the impellers of these pumps are only partly filled with liquid, so tthe power input and temperature rise are about one-third the values for impellers running full with discharge throttling. The capacity of condensate pumps frequently is submerge-controlled, which is equivalent to suction throttling. Special design reduces cavitation damage of these pumps to a negligible amount.

 

[786392]

Thanks Dear 25362.

This is really a C.T.(Confirmatory Test)as referred in chemistry Jargon terms; that such a practice

is adopted in certain Special/critical circumstances of need;

as such may not be in common practice/ usual knowledge,I believe.

Best Regards
Qalander(Chem)

 

[arm83]

Guys,

Thanks for all the responses. You cleared up some issues for me. I was doing this research for a specific reason.

Currently I have three fuel oil pumps that are operated with the suction valve partially opened and the discharge valve fully opened.

This practice developed because the motors are too small. I have to economically justify buying new motors.

The pumps suffer from seal failures only. The pump shows no physical signs of cavitation (from observation of casing and impeller when pump is shopped). The vibrations are under 0.1 ips, the bearing housing temperatures are fine(< 130F)and operations insist that they are getting the flows they require ONCE the suction valve is throttled on start up. The pumps recieve suction from a tank located on a hill and the suction guage which is located before the suction valve reads between 40 to 50 psi.

The seal failures occur every 4-6 months and are catastrophic. Everything from the seal plate to the sleeve is badly damaged. I am sure it is connected to the operation of the pump but I am at a loss of how to prove this. According to the operators, this system has been working for a long time now, why interfere with it?

 

[786392]

The exact type of pump?

Single,Multi stage,Centrifugal Horizontal or Vertical?

Designed Pumping Temp,Pumped Fluid Viscosity range(s),NPSHr and NPSHa?

Any Changes in original Sealing,Seal Oil,Seal cooling Plan?

Any majour changes in Pumped Fluid characterstics?

Any Data Gathered at the failure point with repect to

'Point Pumped Fluid' Conditions/ Charaterstics& NPSHa observed/recorded and Flowrate?

Any changes in No. of pumps in service, whenever Seal Failure witnessed?

Does pumped fluid contain or may contain solids/semi solids in greatly varying %ages?

Failures occur Just after start-up or after long smooth running;abruptly indeed?

The above set of queries and few more may help you diagnose and rectify/trouble shoot the problem.

Hope this proves helpful.



Best Regards
Qalander(Chem)

 

[MikeHalloran]

Well, 50 psi of 'suction' just ain't possible, so I'll assume that's a positive pressure, present because the tanks are pretty far up the hill. Which begs the question of why you need a pump at all.

My other concern is that if the pump seals are designed for an inlet with actual negative pressure and you're supplying a positive pressure, it's possible that a mechanical seal's bellows would be everted by pressure in the wrong direction. Bellows generally don't like that.



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

I like your operators idea of good performance.
Hellooooo? Need to fire those guys.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

"25362 (Chemical) 24 Mar 09 6:48


FYI, this was taken from the Pump Handbook by Karassik et al. (McGraw-Hill) on the subject of suction throttling.
Quote:
...Jet engine fuel pumps frequently are suction throttled because discharge throttling may cause overheating and vaporization --------------
Special design reduces cavitation damage of these pumps to a negligible amount."

All well and good, but notice the 2 important words "SPECIAL DESIGN".

 

[BigInch]

It can be a long time waiting in line (at idle) for takeoff. I suspect the best hydraulic solution would be to recycle to the tanks, but extra weight considerations, or another aeronautic consideration else made that solution inconvenient. Was the potential vaporization caused by in-pump recirculation heating or by flashing across the (rejected) discharge control valve?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[786392]

Anything further from OP for us to know?

Best Regards
Qalander(Chem)

 

[25362]

Artisi, you're absolutely right!
However, let's not forget Linus Pauling saying:

The best way to have a good idea is to have a lot of ideas.

 

[BigInch]

25362
That was obviously before QA/QC became vogue.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/