Pulsation Damping for plunger pump 1

[6748]

We are using a triplex plunger pump for pumping 23 cubic meter per hour of water at 40 Kg/cm2 discharge pressure. The suction is from a discharge of a centrifugal pump operating with a discharge press of 3 Kg/cm2 pressure. The flow fluctuation with this triplex pump will be ie Flow max divided by flow average is 1.045. (As noted from a standard text book). When this pump runs the suction pipeline, bag filter vessel in the suction line, the rotameter in the suction line and all the discharge pipe lines vibrate. The threaded pressure relief valve in the discharge line cracks the female-threaded-socket.

The text book states that if a pressure or flow dampner ( a simple air vessel) of sufficient volume at the suction and discharge lines closer to the plunger pump's suction and discharge manifold is installed the vibrations can stop or reduce. The suction line is 2 inches dia. Discharge is 2 inches dia.

Question No 01.-->The question is how to size the damping vessel?
The discharge of the above triplex pump flows thru a 8 inch dia x 5 mtere length vessel which is maintained at 40 Kg pressure. The discharge pressure is maintained at 40 Kg/cm2 by throttling the discharge flow at the outlet of the process vessel. After this throttle valve, the water flows out at about 1.0 Kg/cm2 pressure. This throttled discharge flow makes a lot of hissing noise. And this almost atmospheric pressure flow in the final discharge line also vibrates.
Question No 02. Do we need a dampner at the outlet of the throttling valve?

 

[jmw]

This has proven a useful web site to me:

http://www.pulsation-shock.com/

 

[bvi]

A triplex single acting pump will have downstream pressure pulsations as high as about 40% of the nominal discharge pressure in the absence of a dampener. You should have the dampener supplier size it.

A suction flow stabilizer may be required to maintain the required pump NPSH, depending upon the suction pressure.

With respect to the noise, make sure that your problem is not cavitation (typically a broad band crackling sound). This can occur if you have a transient pressure drop downstream of the valve, resulting in transient vaporatization and subsequent recollapse of the bubbles. Alternatively, if the bubbles stay as bubbles, you can have two phase flow induced vibration. Note that pressure pulsations can contribute to cavitation (on the down side of the pressure pulse).

 

[abcmex]

some comments:
1. Dampener in plunger pumps are installed mainly in the discharge but they are used also in the suction line. perhaps helps:

http://www.lewa.de/pdf/broschuere/D10-012en-charactericstics.pdf

2. Simple dampeners (air vessel) are effective only where the personnel know how use them. Dampeners with a membrane are much more reliable (and expensive).
3. Is the process vessel installed to work like a dampener?
check the gas volume avaliable to see if this is enough.
4. The pressure drop (40 to 1 barg) is probably causing the noise. Review the calculation of the valve (including noise generation). Perhaps you need a special valve.
RGS

 

[6748]

Dear jmw, thanks for recommending the site.
Dear cb4, The pump supplier has provided a so-called-dampener. Its volume is hardly 2500 cc ie 2.5 litres. In the discharge line the pressure max is 40 Kg/cm2 and the pressure minimum is atmospheric. Even if this small dampener is full of air, the air volume inside the dampener will shrink to around 65 ml. at 40 Kg pressure. I don't think that this will do the job of dampening.
For providing NPSH, we are using a centrifugal pump to feed to the suction of the plunger pump. The suction of the plunger pump is under a positive pressure of 2 Kg/cm2g.
No cavitation can occur, since the centrifugal pump can pump 30 m3/hr against the plunger pump rating of 23 m3/hr.
No flashing can occur since the feed water is at 25 Deg C.
Dear abcmex, the discharge throttling valve is a conventional globe valve. I will be thankful if somone can suggest a suitable valve for reducing pressure from 40 Kg to 1 Kg/cm2 pressure for a noncompressible fluid like water. The pipeline size is 2 inches.

The question No 1. The site that jmw has recommeded talks of a better design dampener that is other than the T-joint air vessels. The present set up is only a small scale pilot plant trial run to understand the process. So for a simple T-joint air vessel dampeners, what should be basis for sizing?

The full plant, in case wo go for it, will have a very high rpm ( 10,000 rpm or so), may be a single stage centrifugal pump for the full flow. But to go for it we need this pilot plant data. This plant is suffering vibration and something or other goes off within 5 to 6 hours of operation.

 

[bvi]

Check the dampener sizing with companies that manufacture dampeners. The theoretical sizes given in text books typically are much larger than those that dampener companies provide, which presumably work. Typically, I would expect a dampener with a bladder with a precharge pressure. Because of the precharge pressure of the bladder, it will not shrink to the degree you state. I still think it may be worth confirming you aren't getting transient flashing in the discharge. You have a large pressure drop. Put your hand on the pipe and see what you feel.

 

[6748]

Dear cb4,
Dampener sizing is being checked with the supplier.
Does this "transient flashing in discharge" mean that the liquid is flashing out at the outlet of the throttle valve that releases the 40 Kg/cm2 press to almost atmospheric. It looks that you may be correct. We have a glass rotameter at the outlet of this throttle valve. It sometimes passes lot of bubbles. We were presuming that those were air bubbles and were wondering "how it entered into a always positively pressured line."
What should be done to minimise/avoid this transient flashing due to pressure reduction?

 

[jmw]

It occurs to me that one solution might be to drop the pressure in stages. 40 to 1 Kg/m2 maybe too much of a step in one go. Using two pressure reducing regulators in series might give each of them a better control condition and help minimise the problem. e.g. 40 to 10 and 10 to 1.
Pressure let-down in stages is important in gas streams, but for a whole loot of other reasons like avoiding condensate. But I'm just guessing here, what does anyone else think?

 

[bvi]

Yes, that is what I mean by transient flashing. You can try another valve that has less of a transient pressure drop. Check with control valve suppliers. Another possibility is to put an orifice plate downstream of the pressure reducing valve to put some back pressure on it. It is something that needs to be designed.