Metering Pump

[mech1122]

According to API 675 section 2.1.15 it states that "Rated Capacity shall be at least 110 percent of the maximum capacity specified".
What does it mean?

 

[bimr]

Rated capacity is the capacity of the pump at the stated operating conditions. I believe that the specification requires the pump selection to have 10% more capacity than the maximum capacity specified.

Your data sheet would show the minimum and maximum operating capacities. The pump should be capable of pumping 10% more than maximum operating. Since the pump is not operating at the extreme design limits, it should have a longer service life.

 

[Pumpsonly]

API 675 is a reciprocating PD controlled volume pump. The capacity of the pump can be varied by the stroke length of the plunger or piston. The rated capacity of the pump is the flow rate at 100% stroke length at the specified discharge pressure. Maximum capacity is the maximum process flow requirement specified by the user. By having the rated capacity higher than the max. required process flow allow user to extended MTBM when the the packing worn out reducing volumetric efficiency of the pump by increasing the stroke length.

 

[RodeteG]

It means that the pump maximum capacity should be 10% higher than your maximum capacity required (just as bimr said), so in fact you are oversizing the pump selection.

If the reason behind this is service life or packing worn I don't know. I will be more inclined to think that is because of pump efficiency (depending if plunger or dipahragm and also viscosity, discharge pressure etc...you may have difference efficiencies around 90-95%)

I don't think is service life, many petrochemical companies already limit the strokes per minute on their specifications for API675 pumps, which in my opinion is the real service life factor.
If it is packing worn and MTBM I assume this could be really important in plunger pump, but, does it affect so much in diaphragm pumps?

Please correct me if I'm wrong!

 

[bimr]

Although a metering pump can generally be adjusted to pump at any flow rate between 0
and its maximum capacity, its accuracy is measured over a range determined by the pump's
turndown ratio.

Metering pump accuracy tends to drop off slightly as the pumping rate approaches either end of the operating range of flows, min or max. Adding 10% capacity increases the upper flow range and since you are now pumping at 85% to 90% of the pump's capacity, the accuracy is expected to be improved.

 

[Pumpsonly]

The efficiency of a metering pump is maximum at 100% stroke length and may not pump any thing at all at below 10% stroke length for high pressure application. At low capacity = short stroke length,the undelivered volume of the liquid in the chamber will expand and reduce the suction in flow.