Low Cost Operation Of Plastic Pump

  •   Most pump problems are due to suction issues. In all the years I have been in the pump business I have only found one instance of a pump problem that was related to the discharge, other than of course plastic pump head that have had a discharge valve shut while the pump was running. Shutting a discharge valve off on a pump causes the fluid remaining in the pump to get very hot and damages the housing, bushings, seals, etc. Hence, the focus of this article will be about proper pump installations in regards to pump suction conditions. I will first cover six basic rules and then some additional thoughts or approaches to insure low maintenance and low cost operation of your pumps.

      Before discussing the first rule about providing sufficient NPSH for the pump, we need to discuss the term the concept of Feet of Head. Pumps don’t suck, rather the pump pushes or throws the fluid out of the pump leaving a partial vacuum. Atmospheric pressure (usually) then pushes the fluid into the pump. For centrifugals, this force is measured in Feet of Head. Atmospheric pressure at sea level is 14.7 psia. At sea level, this can also be stated in terms of 29.94 inches of mercury (barometers) or 33.9 feet of water. Hence, at sea level we can say that a tank has 14.7 psia of pressure on it from atmospheric pressure or we can say that it has 33.9 Feet of Head. The convention with centrifugals is measure pressure in Feet of Head. A quick formula to convert between Feet of Head and psi:

      Where does the 2.31 come from? Divide 33.9 feet of head by 14.7 psia. Water has a specific gravity of 1.0. So the formula always works. If the specific gravity is known for the fluid that is being pumped, pressure gauge readings can be converted to Head in Feet, which is useful for determining where the centrifugal pump is operating on its pump curve. A final note before discussing NPSH. Pressure for centrifugal pumps (inlet and outlet) is measured in Feet of Head and pressure for positive displacement pumps is typically measured in psi. One of the exceptions is for air operated diaphragm (AOD) pumps, which is a positive displacement pump where discharge pressure is measured in Feet of Head.


      Simply put a pump will not operate properly without sufficient inlet pressure, the pump will cavitate. Cavitation is caused by the rapid formation of vapor pockets (bubbles) in a flowing liquid in regions of very low pressure and collapsing in higher pressure regions, often a frequent cause of structural damage to the propellers or other parts of the pump. NPSHR or Net Positive Suction Head Required is the technical term used to determine what pressure energy (in psia or feet of head) is needed to fill the pump inlet and not have the pump cavitate. NPSHR is based on pump design. It is a characteristic which varies primarily with pump speed and the viscosity of the fluid.

      The average pressure (in psia or feet of head) is measured at the inlet port during operation, minus the vapor pressure of the liquid at operating temperature. It indicates the amount of useful pressure energy available to fill the pump. What we are asking is does the system provide enough pressure to fill the pump completely and not cavitate (given the pump design, speed, fluid viscosity, etc.)? The following is brief overview of NPSHA and how it is calculated.

      Where Ha = Atmosphere Head is the head or pressure (pressure is measured in feet of head) on the surface of the liquid in the tank that we are pumping out. In an open system like this, it will be atmospheric pressure, 14.7 psi or 34 feet of water.

      Hs = the vertical distance, measured in feet, between the free surface of the liquid to the centerline of the pump impeller. If the liquid is below the pump, this becomes a negative value.

      Hvp = the vapor pressure of the liquid at the pumping temperature, expressed in feet of head.

      Hf = the friction losses in the suction piping, expressed in feet of head.

      The pump gains additional inlet head or pressure if the liquid level is above the pump inlet or minus head if the liquid level is below the pump. The fluid weight creates the pressure. The pump loses inlet head or pressure from friction loss of the fluid moving through the suction pipe (small pipes or long pipes have a lot of friction). And finally the inlet head or pressure is reduced by vapor pressure. This is an issue if the fluid is evaporates easily or is very hot.

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