Some things can be made to work; but just because it works doesn't mean it's the best approach.
I had a customer in Columbus who was metering vegetable oil to spray on baked goods on a conveyor belt. The excess oil would drain to a pan and was then screened before recirculating to the pump.
Some 28 odd years ago, a surplus Viking H32 internal gear pump was installed with a direct drive 1HP 1750RPM motor. A variable speed drive and flow control needle valve were added to control the flow of the oil.
However, the actual flow needed is 1.5-2GPM and the Viking H32 is capable of 16GPM at 1750RPM even though the max speed for the pump is 1150RPM or about 10GPM. Also, the shaft was not being supported by a bearing bracket which is recommended for this pump and the pump had Bronze Bushings which are not suitable for this vegetable oil service.
To further complicate the problem the variable speed drive could not run slow enough to meter the oil without tripping the motor (horsepower is proportional to speed (Hz)) which left the flow control needle valve doing most of the metering work but this would easily clog with crumbs. The needle valve restricted flow causing the pumps 50PSI relief valve to be continuously open and bypassing, and creating backpressure and increasing power requirements limiting the variable speed drives effectiveness.
As a solution we installed a Viking G32 with carbon graphite bushings and a bearing bracket. We used a 3/4HP gear motor with 410RPM out for 2GPM at 60Hz, and utilized their existing variable speed drive.
This completely eliminated the needle valve and now flow control is done solely with the variable speed drive.
Finally we installed an Eaton duplex strainer downstream to better collect the small crumbs.
