The
Effects of Valve Dynamics on Reciprocating Pump Reliability
S. M. Price, D. R. Smith, and J. D. Tison, 12th International
Pump Users Symposium & Short Courses, Texas A&M University,
Houston, TX, March 1995.
Compared to leading edge technologies such as high speed turbomachinery,
reciprocating pumps may appear to be a simple technology: A reciprocating
plunger causes liquid to be drawn from a low pressure manifold through
the suction check valves, and expelled into a high pressure manifold
through the discharge check valves. However, the interaction of
flow, valve dynamics, and the acoustics of the pump and piping system
can generate high amplitude pressure pulsation, causing severe vibration
and reliability problems in some systems. The effects of improperly
operating valves, and the successful resolution of such problems
with valve design modifications are examined.
Since the
pump valves are operated by the action of the fluid, valve components
(including the springs, valve body, valve disc shape, and sealing
surfaces, etc.) must be carefully selected to ensure that the
valves open and close at the appropriate times. Problems associated
with valve dynamics include over pressure spikes at the opening
of discharge valves, under pressure spikes at the opening of the
suction valves, high noise levels, and excessive wear of valve
components.
Severe over
pressure spikes have been known to cause fatigue failures of pump
working barrels, connecting rods, bearings and even drive-train
components. Under pressure spikes can cause cavitation that can
also lead to plunger and valve failures, due to pitting damage.
High level valve noise is usually indicative of severe impacts
associated with the opening and closing of the valves. Impacts
at the valve closing are sometimes referred to as valve hammer
and can result in damage to the sealing surfaces. Impacts at the
valve opening, which may be identified by damage to the back side
of the valve disc, are due to the over pressure and under pressure
spikes which can result in fatigue failures of components. Excessive
valve wear can also be experienced when the valve disc material
is not suitable for the pumping conditions.
The use of
field data coupled with a computer model to analyze the valve
dynamics will be presented. Using this tool, the effect of various
valve modifications such as changes in sealing surfaces, valve
lift, spring preload, and spring stiffness can be observed. Instrumentation
and data analysis techniques to evaluate these problems will also
be discussed. The effects of modifications to valves (including
changes in lift, spring preload, spring construction and stiffness,
valve disc geometries, etc.) will be examined using data from
actual systems.
|
