How Does Slip Affect Pump Performance?

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In this Pump Report, Chad explains how slip affects a rotary positive displacement pump's capacity. To learn more, visit our website at vikingpump.com.

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Hi I'm Chad Wunderlich with Viking Pump and a few episodes back I talked about how we can calculate the flow rates for rotary positive displacement pumps - like this gear pump. I'd explained that a pump's capacity can be determined by the simple equation of multiplying the pump's displacement by its speed. Well...that's not entirely true. You're watching the Pump Report.

Okay - so by using this equation you can get a quick estimate of a pump's capacity - but that estimate will be slightly higher than the actual capacity delivered by the pump. This is because...of slip. In a rotary positive displacement pump, the flow direction follows the rotation of the gears or other pumping elements. Small clearances between and around the gears allow them to rotate freely but this also allows for a small amount of liquid to leak...or SLIP back from the high pressure side to the low pressure side of the pump. So a PD pump's ACTUAL capacity will be displacement times speed...minus slip. Okay - so I've defined slip and I've told you how it affects a pump's capacity. But how do we determine this value? How much slip is in your pump and how much slip is too much? Slip is dependent on several variables: Differential pressure...liquid viscosity...and clearances. If the pressure is low, viscosity is high, and clearances are tight...slip is low and the actual flow is very close to the theoretical capacity of the pump. Conversely, if the pressure is high, viscosity is low, and clearances are wide...the slip is high. So here I've made a chart of capacity versus speed. Using the equation of displacement times speed equaling the theoretical capacity...we end up with a line that runs through zero - that is the theoretical capacity for a positive displacement pump. When we subtract slip, we can see that the actual capacity runs parallel to that but does not cross through zero. It's important to remember that slip is a constant. It's independent of speed and it stays the same as long as the other variables stay the same. And since slip is constant, it's important to remember that as we move left on the curve and slow the pump speed it becomes a higher percentage of the overall capacity of the pump. You can operate your Viking pump over a range of speeds and flows on the curve. But operating beyond the left end of the curve can result in heating of the pump and it can be difficult to hold consistent flow as even minor changes to our slip variables can have a major impact on the capacity of the pump. So now you understand slip, its effect on capacity, and how it's calculated. So how do you calculate it? Don't worry about it. Viking Pump has already done that math. When you access your Viking Pump performance curve that's going to give you an accurate value for that pump's performance at your operating conditions. To learn more about slip or to view other Pump Reports, please visit our website at vikingpump.com.