7 replies to this topic

I was wondering whether anybody knows where I can find guidence on at what upstream pressures it is necessary to install double NRVs (check valves) in order to provide protection against backflow on positive displacement (injection) pumps. I know that positive displacement pumps only contain very small inherent clearances within them so when they're turned off they will provide some resistance against backflow. This means that for non-critical operation it may be adequate to install single NRVs. It would therefore be useful if I had some information on the sort of upstream pressures will cause leakage backwards through the pump in the event of the failure of the single NRV. I am only after ballpark figures and any information that anybody has would be very useful.

Regards,

Dan

[Art Montemayor]

Dan:

I think that as an engineer it would be more important to first fully understand just exactly HOW a positive displace pump - such as a piston or plunger device - works. Are you talking about a piston or plunger pump?

If so, did you know that ALL piston or plunger pumps have TWO (2) check valves built into the basic design? That is the fundamental reason that they work.

[latexman]

Using just upstream pressure to determine if backflow protection needs improving is too generic in my opinion. I hope you have omitted part of your thought process for expediancy, because the consequences of each unique situation must be considered. Ask the "what if" question. You could have a high pressure water pump connected only to water. What if you get water in the water tank? No problem. You may have a low pressure pump pumping oxidizer into a reactor. What if you get reactor contents into the oxidizer? Huge problem. You see it's not just pressure.

Thanks for your replies. I do realise that piston and plunger pumps require two check valves (one on the inlet and one on the outlet), but maybe I wasn't clear enough. I'm considering placing two check valves in series on the outlet of the pump, therefore in the event of the failure of one check valve the other should provide protection.

The pumps are caustic injection pumps on a petrochemicals plant, therefore backflow is a major problem in this situation and it is an EHS critical task. By knowing the sort of upstream pressures that will cause me a problem I can consider whether the installation of double check valve configurations is necessary or whether I can discount this requirement due to the risk being too low.

Regards,

Dan

[latexman]

My company does not give credit for a check valve in a safety analysis, like in a LOPA. Does yours? Also, the more independent the two layers of protection are, the better. Duplicating a layer of protection, i.e. adding a second check valve, is a weak layer of protection. They are very susecpible to common cause failure. For example, if one sticks open for some reason, how will you know? Check valves have no diagnostics. It will sit there failed and un-noticed until the second one fails, then you have a problem and no warning. A stronger layer would be a low flow shutdown or a low dP shutdown.

[djack77494]

latexman makes several very good points, none of which do I challenge. I have NEVER seen multiple external check valves in the flow path of a PD pump. As Art points out, you already have 2 internal checks before you even buy one. Add an external check and you have three. How many do you think are needed? The answer is that you have reached the point of diminishing returns. If three checks will all fail, then four will as well. For critical services where it is absolutely imperative that there be no backflow, use a backflow preventer. These are common in potable water systems and are very reliable and effective, though I'm not sure about their usage in various services.

[Dexco]

Dan, the best way to have multiple layers of protection on a PD pump is to have a check valve in the discharge line only. Never in your suction piping as it restricts the flow, which is not good (may cause cavitation). I recommend using the Durabla WLC inline wafer style SS check valve. It works very very well. As for a secondary backup, the guys are right, 2 check valves does not provide double the protection. On our pump packages that we manufacture we install Fisher control valves on the discharge line, so when the pump is turned off or there is no flow the actuator closes the valve, thus creating a positive seal against the backflow on the discharge line. When the pump starts up the valve is set to open automatically again.

As for the "check valves" in the pump. Pump valves are not a reliable option to stop reverse flow. For example, if they are at all worn they will not seal 100%. If your packing is also worn it can result in a leak of fluids into the plunger area of the pump and if not properly drained will make it's way into the power end of the pump, you will most likely have some maintenance issues.

Steven
pumps@dexcointernational.com

[Dexco]

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