The pitfalls of centrifugal pump piping are many and varied. However, the main difficulty is that a poor piping layout places the cause of many pump failures outside the physical confines of the pump itself. This makes detection tough for the unwary and inexperienced. Adding to the difficulty, accessible information on the topic of pump-piping design is scarce, and what little is available usually is ignored when a new pump is being installed.
The result of all this is that most pumps are piped incorrectly, resulting in repetitive failures that go unexplained and unresolved. In fact, the way some pumps are installed resembles a "plumber's nightmare." Many pumps are squeezed into a corner and the pipes are threaded in and out without any consideration for fluid flow patterns.
Figure 1:Positioning the reducer with the flat side on top eliminates the potential problem of air pockets at a high point in the suction line.
Many who read this article will realize to their horror that some of the most problematic pumps in their plants don't follow any of the basic rules of pump piping. Why is this?
To answer that question, let's consider what normally happens when we are about to install a pump in a new system. Where do we go for information about how to arrange the pump piping? Most of us would refer to the pump manufacturer's Installation, Operation and Maintenance (IOM) Manual. Unfortunately, that won't provide a lot of information, because most pump makers have restricted their responsibility to the confines of the suction and discharge nozzles of the pump. Although this attitude is fast disappearing, the change has not yet reached most IOM manuals. As a consequence, accurate and complete information is still severely limited. A high proportion of pumps are installed with inappropriate piping arrangements that result in premature failure.
In spite of this, you can easily avoid the common pitfalls of pump piping by following seven straightforward rules.
Rule No. 1
Painful practical experience has shown that if the pump is not easily accessible, it will not get the routine care and attention required to ensure its reliability. What should be a simple matter of checking the running temperature of the bearing housing can often become an unwilling exploration into the depths of a piping jungle. Even keeping the unit clean to facilitate inspection can become an unmanageable chore.
Figure 2:This configuration introduces uneven flows into the opposing eyes of the impeller, upsetting the hydraulic balance of the rotating element.
Rule No. 2
A concentric increaser should be bolted directly to the vertical discharge flange prior to the location of any valves. This increaser will slow down the velocity of the liquid leaving the pump to an acceptable rate within the system itself and, in particular, through the check valve and isolating valve. Keeping the velocity low (usually less than 10 ft/sec) reduces friction losses in the line and power draw at the pump.
Figure 3:An adequate straight length of piping provides uniform flow of liquid to the eye of the impeller and the optimum suction condition.
Rule No. 3
There always is an uneven flow in an elbow. Installing an elbow on the suction of any pump introduces that uneven flow into the eye of the impeller. This can create turbulence and air entrainment, which can result in impeller damage, vibration and premature pump failure.
The problem is compounded when the elbow is installed in a horizontal plane on the inlet of a horizontal double-suction pump, as shown in Figure 2. This configuration introduces uneven flows into the opposing eyes of the impeller, essentially destroying the hydraulic balance of the rotating element.
Under these conditions, the overloaded bearing will fail prematurely and regularly if the pump is packed. However, if the pump is fitted with a mechanical seal, the seal usually will fail instead of the bearing -- but just as regularly and, often, more frequently. When it is absolutely essential to position an elbow on the inlet of a double suction pump, it must be located at right angles to the shaft.
The only thing worse than one elbow on the suction of a pump is two elbows on the suction -- particularly if they are positioned close together in planes at right angles to each other. This creates a spinning effect in the liquid that is carried into the impeller and causes turbulence, inefficiency and vibration.
Rule No. 4
Note that this layout contradicts the information given in most IOM manuals that the suction piping should be "as short as possible." This arrangement also supports Rule No. 1 and should eliminate the practice of squeezing the pump into a corner without any consideration for fluid flow patterns or physical accessibility.