Today, operating companies increasingly are making decisions about pumping systems based on life-cycle costs rather than initial purchase price. As a result, many seemingly insignificant aspects of pump design are starting to come into play more and more.
So, a common question these days is: Why do most end-suction open-impeller centrifugal pumps have back pump-out vanes and what do they do for you?
Quite simply, in such pumps the vanes are vital for minimizing the thrust loads on bearings and the pressure on mechanical seals and thus for lengthening the lives of bearings and seals. They also help deal with solids.
To understand why, lets start with some pump basics.
End-suction centrifugal process pumps whether ANSI B73.1 or larger or smaller sizes have either an enclosed- or open-impeller design. Both styles move fluid and develop pressure, but each has its own unique characteristics. In general, enclosed impellers offer higher efficiency, while open impellers allow a higher solids content. Back pump-out vanes are found predominantly on open impellers.
An enclosed impeller will balance the majority of the developed pressure through the design of the impeller. Discharge pressure seen along the front shroud counteracts most of the discharge pressure on the back shroud. The remaining unbalanced pressure acting on the back of the impeller generates a thrust load towards suction. This load must be absorbed by the thrust bearing. In most cases, this is a moderate load that is easily handled with a standard thrust bearing.
An open impeller is a different story, though. Because the impeller does not have a front shroud, it cannot isolate discharge pressure along its entire front face. So, pressure gradually builds along the face from a low of suction pressure at the eye of the impeller to a high of discharge pressure at its periphery. Without any back pump-out vanes, there is discharge pressure on the entire rear shroud. This results in a larger thrust force for the bearings to absorb.
An important role
By adding back pump-out vanes, the pressure on the back side of the impeller can be reduced. The profile of a back pump-out vane typically is a mirror image of the working vane (Figure 1), although there are exceptions. These vanes act somewhat like an impeller. However, because they are so much smaller, the pressure they develop cannot overcome that developed by the working vanes. Instead, the back pump-out vanes simply act to break down that discharge pressure to a value between suction pressure and discharge pressure.
Reducing the pressure on the backside of the impeller decreases the thrust force. This results in extended life for the thrust bearing because it needs to do less work.
That covers the bearing portion. Now lets take a look at the mechanical seals. For the seals to have a long life, it is imperative that they have a good operating environment. Ideally, this means cool fluid (seals generate heat), no solids and low pressure. Obviously, this environment cannot always be achieved. The application generally determines the fluid temperature and solids content imposed on the seal. However, in many cases, the seal chamber pressure can be adjusted reducing the pressure as much as possible is important for minimizing the wear on the seal faces.
To reduce the seal chamber pressure, you either can vent it out of the seal chamber or somehow break it down to something less than discharge pressure.
The pressure can be vented in two ways:
1. By providing balance holes in the impeller shroud. This allows the higher pressure on the backside of the impeller to vent back to suction through the impeller. The advantage to balance holes is that they are internal to the pump and, except for some minor additional machining, do not require any significant modifications. However, balance holes slightly reduce pump efficiency. For smaller ANSI-style pumps, this efficiency loss typically is negligible. However, for larger-than-ANSI pumps, this loss of efficiency can be significant.
2. Using a seal flush plan specifically, an ANSI B73.1 Plan 13. This is rather simple and effective, but does require additional piping or tubing to be run from the seal chamber or seal gland back to the pump suction.
The alternative method to reduce seal chamber pressure is to break down the discharge pressure. For enclosed impellers, this typically is done using a wear ring. For open impellers, the most common method is via pump-out vanes on the backside of the impeller; this is a simple solution that usually does not require any extra machining. The vanes are internal to the pump, so there is no additional equipment that can be damaged during shipment, installation or operation.
Because back pump-out vanes are inexpensive and add value, most pump manufacturers include them on their open impellers as standard. In rare cases, however, they should not be used when a pump with an unflushed mechanical seal is in lift service and operating near runout, back pump-out vanes may allow the seal to run dry.
In general, back pump-out vanes can break down between 65% and 85% of the developed pressure within the pump by the time the fluid gets to the seal chamber. This is a significant reduction, especially when you are looking at high head services. It can mean the difference between a seal failing prematurely or providing a reasonable lifespan.
In addition, the back pump-out vanes aid in dealing with solids. The vanes will keep the solids in suspension and help to move them out towards the pump discharge. This action will reduce the solids concentration at the seal faces, thus increasing the life of the seal.
A common misconception regarding back pump-out vanes is that increased clearance between them and the seal chamber face equates to decreased performance. This is not true. The initial vane-to-face clearance on a new pump is roughly 0.015-0.040 in. This is relatively large, considering the impeller-to-casing clearance generally is 0.015 in. Testing has shown that the gap does not adversely affect the thrust loads or the seal chamber pressure, regardless of the number of times the impeller clearance has been set.
Back pump-out vanes serve an important, if underappreciated, role.
Richard Nardone is product manager for ITT Industrial & BioPharm Group, Seneca Falls, N.Y. E-mail him at Rich.Nardone@ITT.com