1 reply to this topic

[process.eng88]

Hi,

 

 

I’m a Junior Process Engineer currently working in a petrochemical plant.

 

I am supposed to specify Low Pressure Alarm for discharge of Centrifugal Pumps, which have been installed for Chilled Water Service.

 

 

2 pumps will be in operation and 1 pump will be on standby.

 

The Standby pump should auto-cut in at the Low Pressure Alarm value.

 

 

There is a level of around 15m at all times on the pump suction so the Low Pressure Alarm should be not set at 1.5 barg (15m head) or lower. Rather, the alarm should be set at a suitable margin below normal operating pressures.

 

 

I intend to specify the Low Pressure Alarm on the basis that the motor is not overloaded owing to Total dynamic head becoming lower than pump rating by pumping very high flowrate of water.

 

 

I’ve read on a CheForums tread that damage of pump's motor can be prevented by not operating beyond pre-specified point as "end of curve" on pump curve.

 

 

Q. How can the “end of curve” point be determined for the attached pump curve for the pump’s rated impeller of 380 mm?

 

Q. How can the attached Motor Loading Curve be used to determine the maximum safe range of motor operation (maximum Motor Power value) beyond which motor will get damaged owing to overloading?

 

 

I can use the maximum Motor Power value to determine the corresponding shaft power on pump curve based upon Motor Efficiency.

 

The shaft power on the pump curve can be used to determine the corresponding low pressure head at high flowrate for specifying low pressure alarm.

 

 

The Pump Datasheet, Pump Curve and Motor Loading Curve have been attached.

 

 

 

I would be extremely grateful for guidance and technical assistance as this is my first experience of specifying Low Pressure Alarm for Pumps.

 

 

Please, kindly highlight any mistakes in my approach to specify the Low Pressure Alarm. 

 

 

 DS- Chiller Circulation Pumps.pdf   209.7KB   23 downloads

 Pump Curve.pdf   321.84KB   30 downloads

 Motor Loading Curve.pdf   52.04KB   26 downloads

Edited by process.eng88, 12 November 2014 - 12:22 PM.

[GHartmann]

First, although your datasheet has the normal convention for specifying head, any data sheet that mixes pressures and head will inevitably lead to errors.  I suggest converting everything to meters of fluid handled.

 

You can then find the normal operating point on your curve.  I would then try to determine the pressure at which indicates the first pump is not operating (although you may want to consider low flow as another option).  This will probably be out your curve past the maximum flow condition. You can then determine a amperage for this condition ; an low amperage would indicate a pump malfunction.

 

For your pumps and the 200 kW motor I do not see how you can EVER overload the motor.  Although, I am not sure I understand all the curves on your motor data sheet.

 

Your end of curve point is exactly that; the end of the curve. About 1750 m3/h and 22.5 m for your pump, with about 145 kW ( x .996 ) power consumption.  0.996 is density correction.

 

Hope this helps.

 

G. Hartmann

BS ChE North Carolina State University

USA