Required Inlet Pressure or NPSHR
In my first blog we argued that pumps don’t suck, they actually rely upon atmospheric pressure to push liquid into the impeller. In my second blog, I explained how atmospheric pressure can be expressed in feet of head. In this blog we will discuss how pressure, measured in feet of head, may or may not satisfy a centrifugal pump’s requirement for inlet pressure.
As a centrifugal pump’s impeller rotates the liquid inside is thrown out, or at, the vanes. As this happens a pressure is required behind the liquid entering the impeller eye so as to move the liquid into the space left by the liquid being expelled from the impeller. This allows the cycle to continue as one continuous uninterrupted movement.
If the rotational speed of an impeller was extremely slow than virtually no pressure would be required to replace the liquid as it was expelled. However, if that were the case, the output pressure from the pump would also be extremely low making for an essentially useless pump.
As the rotational speed of the impeller is increased to a useful speed, the requirement for pressure behind the fluid entering the eye also increases.
Should at any point the impeller expel liquid at such a rate that the pressure behind the inlet water cannot accelerate liquid in at the same rate, a “relative void” will be created (I say "relative void" as it acts similar to a void but future blogs will explain its true characteristics). The void, as we will call it for the time being, has no mass and therefore cannot create a force as it passes down the impeller vane. No force equates to no pressure and a pump is useless if it has no output pressure.
A demonstration illustrated by a series of two photographs can be used to explain this ”relative void”.
In Photograph #1, a ball on a probe is slowly being moved through liquid from right to left. As the ball moves, hydraulic pressure forces the surrounding liquid in behind the moving ball, filling in the area before any void is created. This mimics the movement of fluid into the vanes of a slow moving impeller, filling the space created as liquid is expelled by centrifugal force.
Photograph #1 Photograph #2
Photograph #2 shows a visible area void of liquid as the same ball is moved very rapidly from right to left. Clearly the pressure is not sufficient to quickly move the liquid in behind the ball. The void created is similar to the void left in a fast moving impeller vane when the inlet pressure is insufficient to quickly fill the space left by fluid being expelled by the impeller.
Pump manufactures are very aware of the need for inlet pressure to keep a centrifugal pump supplied with a continuous feed of liquid. They calculate and then test every impeller through out its expected operating range and record this information as what they call NPSHR. ( Net Positive Suction Head Required )
NPSHR data is critical information for pump operators in many applications. Manufactures provide NPSHR data to the user in the form of NPSHR curves. One such curve is provided below.
The green line denotes the minimum pressure, in ft or meters of water head, required to properly fill the pump regardless of impeller speed or flow rate.
The steepest curving black line would be associated with the fastest of three operating rpms tested. You will notice that the faster an impeller has to turn to achieve any specific flow rate, the higher the NPSHR. (As the impeller speed increases the time available to fill the void decreases so the pressure must increase to completely fill the void in a shorter amount of time.)
It is also note worthy that once a mid range flow is exceeded NPSHR increases rapidly at any rpm. High flow rates are a result of large volumes of liquid being expelled from the pump impeller in a short period of time. This necessitates high inlet pressures to move this large volume in a short amount of time.
Today’s discussion introduced the concept of NPSHR. In basic terms, the pressure a pump needs to stay operational. In next weeks blog we will discuss its counterpart, the pressure the environment and inlet system can make available to the pump. (Net positive suction head available or NPSHA.)
In weeks to come we will also expand upon the term “relative voids”. Watch for titles containing the terms cavitation and/or vapour pressure. Stay tuned!
Until next time,