5 replies to this topic

[Sridhar P]

please guide me in the following:

In closed loop pumping system say in an HVAC system(expanion tank provided in pump suction),
is that the pump suction and discharge lines should be of same size. Since as the same liquid discharge comes back again to the suction.

Is that pump suction line velocity should be same as discharge velocity

Is that pump vendor will also provide same nozzle size for suction and discharge nozzle for closed loop system.

[ARAZA]

Hi There,

The pump suction line sizing basis is any acceptable velocity range for that diamter pipe plus you would have to be extra careful when calculating NPSH. Higher velocity contributes to higher pressure which reduces the NPSH calculated.

I do not know your system competely but I undestand that your refrigerant may be at saturated temperature and at its boiling point. For such liquids you would have to be very careful while performing NPSH calcs.

As a general guideline, base your calculations on 0.5 psi total presure drop in your suction piping. If it seems conservative, increase the pressure drop to accomodate a reasonable NPSH.

Regards,

ARAZA
drop,

[Sridhar P]

Hi Araza,

Thanks for your inputs. Need few more clarifications

My system fluid is water. This is ciruculated by a pump through VAM-(Vapour absorption machine), where it gets cooled to 6 deg C from 12 deg C. This is circulated to the end users like heat exchangers like air coolers and air conditioning and returns back at 12 deg C.

This system is provided with expansion tank at pump suction to accomodate for thermal volume changes.

My question here is,

01. For open system, pump suction velocity will be normally taken at 0.5 m/sec. But for closed system-since the pump discharge is finally connected to suction, is that the velocity at pump suction should be same at discharge velocity say for example 2 m/s.

02. Is there any criteria, that for closed circuit pumps, suction and discharge nozzles should be of same size.

Regards,
P.Sridhar

[ARAZA]

Please see my comments in red..

Hi Araza,

Thanks for your inputs. Need few more clarifications

My system fluid is water. This is ciruculated by a pump through VAM-(Vapour absorption machine), where it gets cooled to 6 deg C from 12 deg C. This is circulated to the end users like heat exchangers like air coolers and air conditioning and returns back at 12 deg C.

This system is provided with expansion tank at pump suction to accomodate for thermal volume changes.

My question here is,

01. For open system, pump suction velocity will be normally taken at 0.5 m/sec. But for closed system-since the pump discharge is finally connected to suction, is that the velocity at pump suction should be same at discharge velocity say for example 2 m/s. - As I explained earlier, there is no relation of the velocity with the open or closed system. The velocity is dependent on the pressure drop and NPSH calculation (suction piping), noise and vibration.


02. Is there any criteria, that for closed circuit pumps, suction and discharge nozzles should be of same size. No

Regards,
P.Sridhar

[Sridhar P]

Hi Araza,

Thanks. Now Iam clear.

Thanks and Regards,
P.Sridhar

[JoeWong]

P.Sridhar,
What you really need to ensure is that the NPSHa at the pump suction is higher than NPSHr by the pump at maximum possible temperature of fluid.

I believe noise, vibration, erosion is less severe at the pump suction compare to pump discharge as pump discharge size is normally smaller than suction.