Pump is the core device of fluid industry, its function is like the heart of
human body, providing power for the fluid in each device in the system, ensuring
the normal operation of the process. Commonly used pumps are classified into
vane pumps, positive displacement pumps and special purpose pumps. Among them,
the centrifugal pump of vane pump is the most widely used.
During the
operation of centrifugal pump, two problems need to be avoided: water hammer and
lower than the minimum flow. There are many ways to solve these two problems,
but the basic idea is how to configure a suitable system around the centrifugal
pump. In these systems, valves play a very important role, and with the
deepening of the understanding of these two kinds of problems, many special
valves were born.
Water hammer protection and valve for pump
shutdown
Water hammer is a series of water hammer phenomenon caused by the
drastic change of medium velocity in the pressure pipeline. The harm of water
hammer is very big, it can destroy the pump, pipeline and other equipment. There
are many reasons for water hammer in pressure pipeline, such as quick closing of
valve, abnormal pump stop and so on.
Mechanism of water hammer in pump
stop
When the pump stops suddenly due to abnormal reasons, such as power
failure: at the initial stage, the medium in the pipeline continues to move
forward relying on inertia, but the speed gradually decreases to zero; at this
time, if there is a high-low drop in the pipeline layout, the medium will flow
backward to the pump under the action of gravity; When the backflow medium
reaches a certain speed, the check valve at the outlet of the pump will close
quickly. In this way, the speed of a large number of medium reaching here
suddenly becomes zero, causing a sharp rise in the medium pressure - stop pump
water hammer. A large number of literatures point out that the main cause of
water hammer is the sudden closure of the check valve at the outlet of the pump.
However, some studies have shown that although the check valve at the pump
outlet can be cancelled in some cases, in most cases, the anti backflow setting
at the pump outlet is necessary to prevent a large amount of medium from flowing
back into the centrifugal pump.
Measures to avoid water hammer in pump
stop
There are many measures to avoid the harm of water hammer, such as the
installation of water hammer eliminator, pressure relief valve, pressure
regulating tank, etc. the following only introduces two measures that are more
widely used and related to general valves.
1. Setting slow closing check
valve
Slow closing check valve is a kind of check valve which can close
slowly by adding actuator and damper. The figure above shows the schematic
diagram of the water hammer protection system of the centrifugal pump with slow
closing check valve. At this time, the slow closing check valve needs to be used
with the gate valve (on-off valve). When the medium flows backward under the
action of gravity, the check valve closes slowly, which effectively avoids the
water hammer caused by the sudden closure of ordinary check valve. Its
disadvantage is that because the closing speed is slow, part of the medium
inevitably flows back into the centrifugal pump, so the pump may have mechanical
failure.
2. Setting slow closing check butterfly valve
This method is
widely used and effective in large-scale water conservancy system. The slow
closing check butterfly valve is composed of butterfly valve, executive
structure and hydraulic control system. The figure above is the schematic
diagram of the water hammer protection system for centrifugal pump with slow
closing check butterfly valve. The system only needs to install slow closing
check butterfly valve, which can not only play the role of check valve, but also
play the role of on-off valve. When the centrifugal pump is started, it should
be started slowly first and then quickly to ensure the low load start of the
pump; when the pump stops suddenly, it should be shut down quickly first and
then slowly, which can not only avoid the generation of water hammer, but also
avoid too much medium backflow through the pump and cause the mechanical failure
of the pump.
Avoid minimum flow conditions and valves
The minimum flow is
the flow to ensure the normal operation of the pump. If the pump works under the
condition of lower than the minimum flow rate, it will cause noise and
vibration, the performance of the pump will become unstable, and even cause
abnormal cavitation of the pump, reducing the service life of the pump.
Therefore, measures must be taken to avoid the centrifugal pump working in the
minimum flow condition. At present, the common practice is to set the minimum
flow circuit for the centrifugal pump. But from the value of the pump itself,
only for those large flow, high head, high power centrifugal pump set the
minimum flow circuit is reasonable. The minimum allowable flow rate of
centrifugal pump is determined by the pump manufacturer's calculation or
experiment.
The simplest minimum flow circuit only needs to install an on-off
valve in the circuit, such as gate valve. When the pump is about to be in the
minimum flow condition, open the valve and connect the minimum flow circuit to
avoid the pump working in the minimum flow condition.
summary
The basic
structure of the valves described above is the same as that of general valves,
but due to the corresponding improvements made according to the application
conditions, these valves become special valves to solve these two problems.
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