Diaphragm Type Metering Pumps

Air is not the friend of a chemical metering pump. Air is hard to compress, and as a result, very hard to move. Diaphragm type metering pumps do not move air well, if at all, unlike water which is much denser than air. So if your chemical injection system has air in the suction or discharge piping, do not be surprised when your application experiences

 diminished flow rates or even no flow rate at all from the chemical injection pump.

Positive displacement, diaphragm metering pumps work using a check ball valve at the suction end and discharge end of the pump. These allow for a fixed amount of liquid to enter the pumps solution head with each backward stroke of the pump, and for that same fixed amount to then exit the pump with the forward stroke.

If the chemical metering pump’s wetted end has air in it, the stroke of the diaphragm will not move air forcefully enough to unseat and firmly reseat these check ball valves; which results in the liquid in the piping remaining stagnant. Air keeps the chemical metering pump’s wetted end from displacing liquid.

Until liquid is forced inside, or the air is bled out of the system, the pump will sit there and stroke forever when air locked.

Things to do and look for to avoid air locking your chemical metering pump

If you are having issues you believe to be an air locked system there are several common issues to look for to fix, or eliminate air in the system as the cause of decreased or no output.

  1. Madden chemical metering pumps are self-priming; but to a point. Chemical metering pumps need a moment to fill their solution heads with fluids and then to displace all the air out of the lines. If it is already piped to the pressured system it will be dosing into, the pump WILL NOT self-prime. You would need a valve somewhere in the line to bleed out the air; or, manually prime the line.
  2. Look for defects in the pipe and/or hose. Also look for points that rise and fall in the system. Piping that does not slightly rise throughout the system is susceptible to pockets of air forming in theses rises and falls in the chemical metering system. Pockets of air can air lock your system.
  3. Don’t confuse “dead heading” with air locking when your chemical injection pump is dosing against a pressurized system. Dead heading the pump will cause damage to your dosing pump, rather than simply affect performance. Dead heading is when the pump cannot overcome pressure, or there is blockage in the discharge piping, and the pump either seizes up or the motor can even burn up. If you know your pump is rated to pump against the pressure you are currently seeing on a gauge, odds are much better that any issues are due to air locking, not dead heading.
  4. Occasionally it is just a matter of time. Small positive displacement, diaphragm metering type pumps may take a while to prime up and push the air out of the piping. If the chemical metering pump is big enough (doses at higher flow rates), usually small amounts of air will get pushed right out as long as the chemical pump’s wetted end is filled with liquid. However, with smaller metering pumps, the air may remain in the line forever hindering performance.


Usually, air locking can be avoided by proper start up techniques. See Madden’s operation manuals for our recommended start up procedures. But for any reason, if your system is air locked, hopefully now the reader better understands how to identify and fix the issue.

If you need more help with your chemical metering pump, or want to inquire about purchasing a chemical metering pump for your application, please reach out to a Madden representative near you, or contact the factory directly. You can call (800) 369-6233.

Thank you for reading our article, and have a great day!

2 Replies to “What it means when a chemical metering pump is air locked”

  1. […] The main point of this article is, if you notice your pump is getting hot, don’t panic, it is very likely that your chemical injection pump is fine, and it is just working harder than it should be to pump chemicals at the needed flow rate. It’s more often than not, specifically an “air locked system” which you can read more about in this article. […]

  2. […] of air in order to prime up and start dosing the chemical again. In a previous blog we covered “air locking” extensively, check it out if you want to learn more about this issue with chemical metering […]

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