Written by Rick
Too High Positive Inlet Pressure to the Pump
This is the first article from the cycle – 12 ways how to kill your Diaphragm Pump. Contrary to the article title this is something, that nobody wants to do. In our articles we want to point out problems which may damage your pump, installation or significantly reduce lifetime of wearing parts. Diaphragm Pumps are widely used in transfer applications. To pump fluid from tanks storage location to the production area.
Attention should be paid to this type of installation due to limits on the amount of positive inlet pressure to the pump. Particular attention is required in those cases in which PTFE diaphragms are used. Typically, PTFE diaphragms can handle maximum 0,6 bar of positive inlet pressure (Net Positive Suction Head – NPSH).
When the AODD pump is located at the bottom of the storage tank is very easy to reach and overcome the limit of permissible positive inlet pressure. NPSH should be carefully calculated taking into account the height of the storage tank and the specific gravity of the fluid. Those two combined factors produce a static inlet pressure.
Often this fact is neglected when building an installation and choosing a pump. Specific gravity of many fluids significantly exceeds specific gravity of water (all information about pumps performance or limitations given by pumps manufacturers are always based on water – specific gravity 1g/cm3). For example: Sulfuric Acid 98% – 1,835 g/cm3, Nitric Acid 70% – 1,42 g/cm3, Phosphoric Acid 90% – 1,75 g/cm3, etc. At the 80% of maximum positive inlet pressure diaphragm lifetime will start to shorten significantly.
The real problem starts when pump is running. Each time the ball valve is closed, the fluid velocity created from the inlet stroke is abruptly stopped. This creates a water hammer effect which produces high pressure spikes, and Diaphragm Pump can create pressure spikes up to four times the static NPSH pressure. With such a force in action PTFE diaphragms lasts day, maximum weeks. Diaphragm failure can cause pump damage when fluid enters dry part of them pump. And due to the fact that many fluids are highly aggressive such a failure can also cause significant damage to the facility and be harmful for people.
Overall financial costs as a result of diaphragm failure could be very high. Fluid inlet positive pressure can be very destructive. When is close to the diaphragm permissible limit, shortens its lifetime radically. When exceeds limits, pump and installation can be damage and people at risk.
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