To Check or not to Check?
I often receive questions as to whether a check valve in a pumping system is a good or a bad thing. Auto closing check valves are often an integral part of a successful pumping system; however, they can also be the root of many system failures. The question then becomes “when is it appropriate to incorporate check valves in a piping system?” Answering this question is often different for systems handling clear liquid or non-settling slurries than it is for systems handling settling slurries, so let’s look at both.
Clear liquid or Non–Settling Slurries
Seven factors to consider when deciding if a check valve should be installed:
- If the piping system has a significant elevation change, then back-flow through the pump on system shut down can spin the pump backwards damaging the pump or the pump driver.
- If a pump is operating in parallel with other pumps and there is no check valve present, a reverse flow could be induced when a single pump is shut down or fails. The reverse flow may spin this pump backwards and damage it or the pump driver. In addition, flow diverted backward through the idle pump would further reduce the production within the overall system.
- If the system incorporates a long pipeline, the significant friction component within the system head will be absent on an empty pipe start up. This will cause the pump to operate at run-out conditions as the pipeline is filled. A check valve can limit this problem to the initial charging of the line and thereby reduce possible damage due to cavitation. It can also similarly reduce issues with the pump driver, if the driver was not sized to handle the high-power draw that run-out conditions create.
- Centrifugal pumps have to establish flow through the pump to develop discharge pressure. Pumps operating against a high static closing pressure on the check valve may not be able to move enough liquid through the impeller to open the check valve and establish flow. This circumstance would necessitate the use of a relief valve between the pump and the check valve and for the relief valve to be temporarily opened every time the pump is started. This of course adds another layer of complexity and cost to the overall system.
- Obtaining a check valve with a pressure rating suitable for ultra high lift series systems may be difficult. For example: a deep open pit mine may have over 1000 ft of back-pressure on the primary pump’s check valve if the check valve at a later lift station fails.
- In locations that are subject to freezing weather conditions, dump valves must be installed to drain the line after a shut down when check valves are present. These need to be automated or stringent procedures in order for 24-hour monitoring to be in place.
- Auto check valves that allow some degree of back-flow before totally closing can be the source of catastrophic water hammer.
Settling Slurries
The factors to consider for settling slurries are the same as the ones listed above, but the issue of solids settling in the line and the possibility of a plug forming can add further complications. If check valves are employed, procedures for line flushing must be in place. In addition, dump valves are strongly recommended so lines can be drained of solids should an unscheduled shut down suddenly occur.
My Advice
With so many factors to consider there is no hard and fast rule as to whether a check valve will help or hinder operations. My advice is to:
- Look at your system without check valves and decide if any of the factors listed above are a major concern.
- If there are concerns, look at installing check valves and make up a list of automation and or procedures that would be practical and needed to address those factors.
- The final step then becomes a cost / risk analysis as to whether the automation and or procedures will appropriately address the concerning factors.
I trust that this blog will help some of you decide if and how automatic check valves should be installed. For those readers who were looking for me to simply say “yes or no to check valves”, I apologize!!
RJ
