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Pump Suction Pressure - Chemical Injection Skid

chemical injection pump suction npsha

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#1 Sherif Morsi

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Posted 15 November 2016 - 06:33 AM

Hi,

 

We are considering a major overhaul for a chemical injection skid. The tanks are 2.5 meters high with the low low level alarm set at 200 mm. The injection pump are installed at an elevated level of 100 mm above tank bottom, so the suction line goes up around the same height.

 

1. If the suction pressure at the pump suction nozzle is around 70-80 kPaa. is that ok given it's below atmospheric pressure yet not complete vacuum?

2. Can I account for some syphon effect in the suction line since the line starts from the bottom of the tank, extends to the skid limit then goes up to the pump suction nozzle?

 

Thanks 

Sherif



#2 senthil13

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Posted 15 November 2016 - 08:24 AM

Couple of questions 

 

- Do you have a system in place and i assume you are revamping for some reasons? How does the system perform? Is it being revamped due to some operational issues or expansion? please clarify.

- What type of pump is it? Can they operate at that low NPSHa?

 

A sketch would be beneficial too.



#3 Sherif Morsi

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Posted 15 November 2016 - 09:16 AM

The system is already in place, we are replacing the 20y old tubing with new ones and changing some the connections between the tanks and pumps to meet the new chemical requirements.

 

See attached sketch. I am looking for the old datasheet as we speak 

Attached Files



#4 senthil13

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Posted 15 November 2016 - 04:44 PM

Thanks for the sketch.

 

Considering the location of the pump relative to the tank. you don't have much head to play with. If your new requirement is to increase flow rate. I would consider a NPSH evaluation of this system and possibly increase suction pressure or relocate the pump.

 

You will gain some siphon effect. personally i wouldn't consider any head for this.

 

 

Just my 2 cents..



#5 sunny143

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Posted 16 November 2016 - 03:06 AM

Sherif,

 

As i dont have sufficient information about the tank & pump foundations. Assuming that tank foundation is about 300mm high, instead of relocating the pump, decreasing pump foundation height will be good. This will give marginal head to the pump.

 

Regards,



#6 Sherif Morsi

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Posted 16 November 2016 - 04:01 PM

The flowrate hasn't changed. All what is happening is replacing the old tubes with new ones and re-routing some others from one pump to the other to allow some more flexibility.

 

These pumps have been running for the last 20 years with this arrangement.

 

The other question is if the suction pressure is 80 kPa, would that be a problem for the pump?

 

Sherif 



#7 S.AHMAD

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Posted 16 November 2016 - 04:41 PM

Hi Sherif, salam.

1. Ensure tat the suction pressure is well above the vapor pressure of the fluid being pumping at the fluid temperature. Otherwise the pump will not pumping the fluid accordingly.

2. The energy balance calculation  includes syphonic effect. No need to do a separate calculation.

 

Hope the simplified comments helps.

 

s. ahmad


Edited by S.AHMAD, 16 November 2016 - 04:43 PM.


#8 Sherif Morsi

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Posted 17 November 2016 - 10:02 AM

Hi Ahmad,

 

I don't understand point 2. Would you please elaborate/give more details?

 

Sherif



#9 S.AHMAD

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Posted 17 November 2016 - 10:05 PM

Hi Sherif.

My suggestion is that you do the energy balance then you will understand the point 2.

 

Energy balance:

Vessel pressure + differential elevation head of liquid level - line pressure drop - pump suction pressure = 0

 

Syphonic effect is included in this energy equation. hoiwever, use consistant unit in the aboive simplified energy equation.

 

s.ahmad


Edited by S.AHMAD, 17 November 2016 - 10:13 PM.





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