What's The Difference Between A Flush Line And A Suckback Line?

As you might already know, internal gear pumps are used in a wide variety of applications. Usually the product flows through the pump and lubricates all of the required areas for long life. However, in some cases, we need to make modifications to the basic pump design to get flow into different areas. In this post, we will discuss flush lines and suckback lines or grooves which are used to change pressure in the stuffing box and promote flow.

Mechanical seals and packing require some amount of fluid movement in the stuffing box area to remove heat and lubricate the rotating sealing surfaces. However, some applications and processes have the tendency to build up product that will stagnate in the stuffing box. If you don’t account for this, you may have a premature seal failure or packing cutting into the shaft. By manipulating the box pressure with either a flush or suckback line, we can usually get that fluid moving to provide long-term seal life.

A flush line is like a traditional API Plan 11, where there is a line from the discharge to the stuffing box area. A suckback line or groove can be compared to an API Plan 13 where there is a line from the suction to the stuffing box area. It is important to know that sometimes casings do not have enough stock in this area to permit drilling a hole, so a suckback groove is machined instead. A line or hole is preferred whenever it can be used because it is less expensive, but a groove can be added when needed.

With no flush plan, the stuffing box pressure is approximately 70% of discharge pressure. With a flush line, the stuffing box pressure is 100% discharge pressure. With a suckback line or groove, the stuffing box pressure is equal to the suction pressure. Both of these methods promote additional flow, making it less likely for product to stagnate and cause problems.

Without implementing a flush or suckback line, it’s important to know the issues you might face.

1. If you’re pumping chocolate, corn syrup, or thermosetting resins, there are conditions that might solidify the product behind to the rotor and stall the pump.
   
   
2. If you have a packed pump that is handling relatively thin liquid and is pumping against some pressure, it could excessively leak through the gland.
   
   
3. If your pump has a mechanical seal mounted in the area behind the rotor, you could have a leakage problem because of heat buildup, liquid stagnation, or an accumulation of solids.