4 replies to this topic

[rakchem1980]

Hi All,

 

I am working on ammonia dosing skid for one of our client. We are facing negative NPSH issue in Ammonia transfer pump selection.

 

Process parameters & client’s requirement is given below. 

 

Please provide us the suitable pump maker (pump model) and appropriate solution to overcome from negative NPSH requirement. 

 

Ammonia Dosing Skid

 

1. Item Name: Ammonia transfer pump (positive displacement pump-double diaphragm type)

2. Service: Ammonia solution (25-30% by mass ammonia in water)

3. Pump capacity (max.) : 2 m3/h

4. Height of liquid level in suction vessel above/below pump centre line (min.) : 0.5 m

5. Pressure in suction vessel : 1.013 bar(a)

6. Pumping temperature : 55°C

7. Density at pumping temperature : 980 kg/m3

8. Viscosity at pumping temperature : 1.2 cSt

9. Vapour Pressure at pumping temperature : 1.17 bar(a)

10. Minimum available NPSH : minus (-) 2.36 mlc

11. Total maximum discharge head : 22 m liq.abs

12. Suction line size: 2”

13. Suction fittings: ball valve (02nos); Y-type strainer : 01 nos; reducer : 01 nos

 

Additional client’s requirement:

 

1. Double diaphragm pumps shall be provided with a diaphragm rupture signaling system

2. The positive displacement pump must be a metering type with stroke adjustment to vary the chemical injection rate.

3. Stroke adjustment shall be used for capacity control. Stroke adjustment should be done manually

4. A set of two (2 nos) Ammonia Solution Transfer Pumps of controlled volume positive displacement, double diaphragm type and their motors. 

5. One (1 nos) Ammonia Solution Permanent Container of 1500 Liters in volume.

6. The pump suction line should be made as short and direct as possible to ensure that the required acceleration head is minimized.

 

The negative NPSH is only due to the high vapor pressure of ammonia at 55 deg C.

 

Possible solution to make the positive NPSH is to increase the static head, But this solution is not accepted by client.

Can anybody please help me out on this issue.?????/

 

Regards,

Rakesh Kumar

 

[breizh]

hi ,

can you cool the stream prior to pump it ?

 

Note : I believe this technology of pump is not the right one for this application . If the diaphragm is damaged you will have to manage the spill , the odor ,etc

 

my 2 cents

Breizh

Edited by breizh, 04 June 2017 - 03:06 AM.

[Mahdi1980]

Hi,

 

Are you sure that the NPSHA is -2.36 m in your case?

I believe that it is something around (-)1.3 m according to your data.

Anyhow, in addition to Breizh's comment, You may select a pump with NPSHr as low as possible.

[rakchem1980]

Hi Breizh,

 

Thanks for the reply.

 

The possible alternate solution what i am thinking for making positive (+) NPSHA are listed below:

1) Increase the static head

2) Increase the tank pressure by N2 blanketing

3) Cooling jacketing of tank

4) Cold insulation to tank

If these above solution is not accepted by client (as cost point of view) then please advise what is the possible solution in this case OR

Is there any PD pump vendors which can full fill the client's requirement of NPSHA = minus (-) 1.3 mlc????

Affinity law is applicable for centrifugal pumps. Can the same affinity law is also applicable for positive displacement pumps. Please advice.

 

Hi Mahdi 

 

Thanks for the reply.

Yes, if we removed the friction loss & strainer pressure drop from the NPSHA calculation. Then the NPSHA comes out to be

minus (-)1.3 mlc (piping material stainless steel) as calculated by you. 

As you suggested to select a pump with NPSHr as low as possible.

Can you please advice any PD pump vendors which can fullfill the client's requirement of NPSHA = minus (-) 1.3 mlc????

 

Regards,

Rakesh Kumar

[Mahdi1980]

Hi Rakesh,

 

Anyhow, the client should cooperate with you to overcome this issue.

 

I did not say that you should buy a PD pump which has -1.3 m NPSHr. I mean as a method, a low-NPSH pump can help.

I suggest you to provide a detailed data sheet for the pump and let vendor to recommend depending on your application.

 

By the way, have you checked that if the Ammonia can be transferred by other types of the pumps, for instance, centrifugal one?

In some cases by providing an inducer at the suction or using VS pumps, the NPSH is compensated.

Edited by Mahdi1980, 07 June 2017 - 08:42 AM.