2 replies to this topic

[Gagandeep Singh]

We are currently specifying a VFD pump for unloading LNG from a Road Tanker. The Pump capacity is approx. 100 gpm with differential head of approx. 10 bar, Pump rpm ~ 6500 rpm and discharges to a vertical LNG Storage Vessel.

 

The operating Pressure of LNG Road Tankers would vary depending on the distance covered and BOG generated due to heat leak plus other variables. Similarly, the operating pressure of the LNG Storage Pressure Vessel (MAWP of 18 barg), would also vary depending on top/bottom loading of LNG to vessel and the operating condition.

 

Can we specify a centrifugal pump for this service instead of a VFD Pump?

Generally, the VFD Pump is costlier and we have got vendor offers only for VFD Pumps based on enquiry for above specifications mentioned for Pump.

[thorium90]

Isn't the VFD implying the drive of the motor is variable speed? Centrifugal is the type of pump. You can have a centrifugal pump that is driven by a motor that is run by a VFD.

[Art Montemayor]

For the 55 years I spent in engineering we always called variable frequency drive (where the rpms of the pump is varied through frequency changes) pumps "VFDs".  Now you come along and distinguish a so-called VFD as different from a centrifugal pump.  The term VFD pertains to the type of DRIVE on the pump, not the type of pump.

 

There are basically two types of pump: dynamic (centrifugal) and positive displacement.  I've used both types on cryogenic services - stuff such as liquid Oxyygen, liquid nitrogen, liquid Argon.  I've never used a VFD on those centrifugal types.  I've used variable speed on positive displacement on the atmospheric liquefied gases.  I've used centrifugal (submerged) LNG pumps at constant speed - but never VFD.

 

Could you please explain yourself better.  Perhaps you don't have field experience with pumps.  Why would you want to vary the speed - and therefore the capacity and head - of a centrifugal pump?  I think we now know you are merely transferring the LNG from a road tanker to a fixed storage vessel.  Please tell us the conditions - pressures, capacities, and specific type of pump with a detailed sketch.   That would help to define what you have.