These forces can have destructive effects if not carefully considered in piping design.
by Jack Claxton, Patterson Pump Company, A Gormann-Rupp Company
November 16, 2015

The reaction load R can now be compared to the rated nozzle load, and the deflection dp can be evaluated. If this analysis method is applied to an installation that uses nozzle loads that have been based on a maximum allowable deflection criterion, the piping design should be such that the value of R does not exceed the nozzle load and the deflection dp does not exceed the maximum allowable deflection criterion using Equation 8.

If this analysis method is applied in an installation that does not use nozzle loads that have been based on a maximum allowable deflection criterion, the piping design should ensure that the deflection dpipe from the piping alone using Equation 9 does not exceed 0.005 inches for best results.

dpipe = Fp/kpipe
Equation 9

However, such a piping design can be difficult to achieve, and a deflection of 0.010 inches is unlikely to harm the pump. In this case, the deflection dp at the pump flange will be less than the deflection dpipe from the piping alone because of the resistance to deflection contributed by the pump.

By this analysis, axially rigid piping keeps reaction loads on pumps at manageable levels. An axially stiff piping run usually produces lower pump reaction forces, while axially flexible piping (especially using flexible fittings with no thrust rods or weak thrust rods) usually creates higher pump reaction forces to restrain the force produced by pressure.

In axially weak piping systems, a relatively strong pump structure produces potentially excessive nozzle loads if the deflection is held to acceptable levels, and a relatively weak pump structure is needed to hold the nozzle load to acceptable levels, however, the deflection is excessive.

Axially strong piping systems better enable the objectives of acceptable nozzle loads and deflection to be achieved. These insights underscore the need to coordinate the piping and pump so they successfully interact.

If an elbow at the end of the piping run results in an orthogonal run of pipe, the effects of pressure reaction forces may be evaluated by using the effective piping stiffness for that run of pipe (see Figure 4).

In this case, the stretch of the offset pipe is calculated using the equations for Fp and kpipe to reflect the characteristics of that run of pipe.

dpipe = Fp/kpipe
Equation 10

The resulting deflection dpipe can then be divided by the offset distance. If this resultant is less than 0.001 inches per inch with a maximum of 0.010 inches when multiplied by the pump flange diameter, then the effect of this force is equivalent to the flange parallelism criterion for mechanical pipe strain and is acceptable.

Read the second part of this article here.

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