DLM reports that it has often received calls where valves are blamed as being faulty and not functioning correctly. While the company still takes 'faulty valve' calls very seriously and investigates them immediately, the decades of experience that its engineers hold lead it to believe that this is seldom true.
DLM reports that it has often received calls where valves are blamed as being faulty and not functioning correctly. While the company still takes 'faulty valve' calls very seriously and investigates them immediately, the decades of experience that its engineers hold lead it to believe that this is seldom true. Quite often such calls result in lengthy site visits, as experience tells us to review the design and layout of the reducing valve station. The apparent valve malfunction is often a symptom caused by an oversight in the system layout.
Preventing the problem occurring from the outset is naturally cheaper than investigating the problem and then having to modify the system once it is fully installed. This inconvenience usually also necessitates a period of production inactivity whilst being corrected.
Pressure reducing valve stations have hundreds of possible causes of malfunction. In this article DLM reviews some of the more common areas regularly encountered by its engineers during site visits.
Check the sizing of the reducing valve
a. If the valve is too small, insufficient flow will be achieved to maintain the required outlet pressure and the valve could generate excessive noise.
b. If the valve is too big, it will open and close repeatedly, which will eventually lead to seat damage and leakage. This leakage will get progressively worse until it appears that the valve is not closing.
Check the setting of the reducing valve
a. Other than pilot operated pressure reducing valves, a reducing valve should be set to close approximately 100 kPa higher than the required flowing outlet pressure. However, the valve can be sized for different rises to close. This rise is required to move the valve disc from its open to closed position. If a large rise is allowed, then a smaller valve may be selected. Conversely, the smaller the rise required, the larger the valve needs to be.
b. When selecting the correct spring, the rise to close must be added to the following pressure and the result (set pressure) must be within the spring range.
c. When checking the set pressure, the outlet pipe needs to be closed. When opened, the set pressure is taken to be the pressure at which the valve begins to open.
Avoid large pressure reductions
a. While the maximum pressure reduction will vary according to the type and size of the valve, it is suggested that reductions of more than 10 to 1 should be treated with caution.
b. If large pressure reductions are required, two valves in series should be used, with an intermediate pressure selected.
Check for large flow variations
a. Some valves will handle large inlet flow variations better than others. Generally, the valve size will be selected to pass the largest flow, therefore when it is expected to handle only a small flow, it will be achieved with very little valve lift. This will possibly lead to the valve opening and closing in a cycle called 'hunting', which will eventually lead to seat damage/erosion due to high fluid velocities. Such leakage will get progressively worse until the valve appears not to be closing.
b. If the valve is expected to work with approximately only 10% of the total capacity for prolonged periods, it is suggested to use two unequally sized valves in parallel. The smaller valve should be set at a slightly higher pressure.
Check the selection of the pipe size
a. Generally there are two types of application: liquid and vapour. For simplicity the term 'vapour' is used to refer to many/any air, gas or steam application.
b. On liquid applications, the pipe size is generally the same or larger than the reducing valve. It is also the same size on both the inlet and outlet. This is because liquid is non-compressible, therefore the same inlet/outlet pipe size can pass the required flow.
c. On vapour applications, the pipe size is generally larger than the reducing valve on the inlet, as the valve is very efficient. It is even larger on the outlet. This is because vapours are compressible, hence the higher the pressure, the smaller the volume the vapour requires. Conversely, the lower the pressure the bigger the volume required, hence small pipe on the high pressure inlet and large pipe on the reduced low pressure outlet.
Check the selection of the isolating valves
a. When globe valves are used these should generally be equal to the inlet/outlet pipe size and not the valve size, if it is smaller. Globe valves can cause turbulence, which in turn can cause the reducing valve to become unstable in operation; hence we would not recommend their use.
b. When full bore parallel slide valves are used, these should generally be equal to the valve size, giving potential cost savings.
Check for flow restrictions immediately before and after the reducing valve
a. Restrictions immediately before reducing valves can cause turbulence, which in turn could cause the reducing valve to become unstable in operation. It is important to design out such restrictions or consider fitting a balance line with pilot valves.
b. Restrictions immediately after the reducing valve will cause the pressure signal to the pilot, diaphragm or piston to become erratic. The reducing valve may become unstable in operation.
Check the system for cleanliness or fit inlet strainers
a. The most common cause of seat leakage is dirt on the valve seat.
b. All new installations must be cleaned and flushed prior to the installation of the valve. This will remove dirt and pipe scale, which will have accumulated during construction.
d. It is recommended that a strainer be installed in the inlet pipe, to fully protect the valve seats.
Check for boiler carry over or condensate on steam systems
a. Slugs of water should not be allowed to accumulate, as they will be pushed through the pipework at roughly the same speed as the steam. This is often 10 times that of a normal water system.
b. Adequate steam traps and draining points should be fitted throughout the system to prevent slugging. Inadequate trapping can allow the water slug to impact heavily on the valve and pipework, causing potential lethal damage in extreme cases.
c. Avoid condensate in a steam system, as this will cause excessive erosion of valve components.
d. Under no or low flow conditions, condensate allowed to accumulate in the system can seriously affect the performance of pilot operated reducing valves. The condensate can create an hydraulic lock in the pilot, which causes potential loss of control of the outlet pressure.
Considering these points at the design stage may eliminate problems associated with reducing valve stations. Running through this checklist, may lead to the identification of a problem quickly, without incurring further costs. Dupleix Liquid Meters (DLM) is the sole supplier of the Bailey product range in South Africa, and has been providing over-pressure protection; flow, level and density measurement; automation and control solutions for over 50 years.
For more information contact Gail Wilson, DLM, 011 457 0500, [email protected], www.dlm.co.za
| Tel: | +27 11 457 0500 |
| Email: | [email protected] |
| www: | www.dlm.co.za |
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