What is a Suckback on a Gear Pump?

As fluid moves through gear pumps, from the suction port to the discharge port, it’s natural for a small amount of fluid to slip or seep through internal clearances to the backside of the rotor. In most applications, this isn’t a problem. But in others, this pressurized slow moving fluid can cause some rather annoying issues.

Typically, the path of the slipped fluid is simple. A small amount of fluid slips through internal clearances at the discharge port and finds its way back behind the rotor. The fluid then travels behind the rotor to the suction side of the pump, where it again, makes its way through internal clearances and back into the pump. This process is illustrated here. 

This process is largely unnoted in many circumstances, but as stated earlier, there are some scenarios where this small amount of slip can be a real pain. Here are some examples.

CHOCOLATE (AND OTHER TEMPERATURE SENSITIVE FLUIDS)

Chocolate is one of the most difficult fluids to pump, so naturally it makes the top of this list. Because the fluid moves slowly behind the rotor, it sometimes has the opportunity to solidify and stall the pump.

Of course this is true for other fluids that thicken or harden as they cool, such as corn syrups, adhesives, resins, etc.

PACKED PUMPS + THIN FLUIDS

When a packed pump, handling relatively thin liquid is dealing with slip, you can just about guarantee you’ll have excessive leakage through the gland. Because the slip behind the rotor is under pressure (suction pressure + 50-70% of the differential pressure across the pump), the fluid seeks the path of least resistance. In this case, out the packing gland.

MECHANICAL SEAL ISSUES

It’s not just packing that has potential to let slip through. Mechanical seals will do the same if they are having issues due to heat buildup, liquid stagnation, accumulation of abrasives, or any other myriad of circumstances that cause mechanical seals to leak.

REDUCE OR ELIMINATE SLIP ISSUES WITH A SUCKBACK

So what’s a suckback? A suckback is a hole drilled in the casing from the suction port to the area behind the rotor. Its function is to promote additional flow through the area by reducing pressure. Specifying a suckback on a new pump (or installing one on your current pump) helps:

• Keep fluid moving so it has less opportunity to set up or harden
• Packing box see same pressure as behind the rotor, reducing leakage

If you think your pump needs a suckback, do NOT make your own. Consult an engineer first to make sure this alteration is right for your application, and trust the modification to the manufacturer or service shop that is authorized by the manufacturer.

Having issues with leaking gear pumps or product build up behind the rotor? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin and upper Michigan.