When sizing a sanitary pump, one of the most critical things to consider are the power requirements of the pump. Determining the amount of power a pump needs to impart into the fluid isn’t always as easy as it looks. Too little horsepower and the pump will kick out, too much and your upfront costs and electric bill will be higher than they need to be. And a motor only has one horsepower, so what do brake, work, and viscous horsepower mean? This post will take a look at these and give some insight into how we determine the correct motor size for your sanitary pump application.
To begin, the equation for total pump horsepower required is given below:

BHP= VHP + WHP

Waukesha Universal Pump Sizing Programs Showing Horsepower Requirements

Where BHP is the brake horsepower, VHP is the viscous horsepower, and WHP is the work horsepower. BHP is the total system energy requirement. It is the energy lost in the pump and system that we need to overcome. As I just stated, there are two places where we lose energy- in the pump itself and the system.

The first place we lose energy is in the pump itself. It takes some energy just to turn the shaft or to spin the impeller. This is known as the viscous horsepower, or VHP. VHP is the power required to turn the rotating parts of the pump against the fluid inside of the pump. Viscous horsepower is determined by the design and speed of the pump. We see different VHP’s for different pump types.

For instance, pumping 5 gpm of a sugar solution with a viscosity 10,000 cps with a Waukesha Universal 1 against a discharge pressure of 100 psi yields a VHP of 0.85 while pumping the same solution under the same conditions with a Waukesha Universal 2 pump yields a VHP of 1.79. This is due to several factors, one of which is the more robust shaft design of the Universal two which makes it better suited for higher pressure differentials and some of the other features detailed in previous posts.
WHP, as stated above, is the work horsepower. This is the power required to overcome external system conditions. Sometimes known as fluid horsepower, this is the power we need to overcome system friction and other sources of losses such as heat exchangers or strainers. Hopefully this is a pretty straightforward concept- contact us today if you need help calculating your friction loss.

Now that we know what BHP, VHP, and WHP are, we can better understand the efficiency of a pump as well as the total power requirements. The efficiency of a pump is the ratio of the WHP to the BHP or total horsepower consumed. This should make sense because it shows us what fraction of horsepower we’re losing to the system and what amount of energy we’re losing to the pump. Understanding what are efficiencies are and how we can change improve them can help us make the best pump selection for each individual application.

So if after reading those post you have any additional questions about the horsepower requirements of your system, contact a Holland Sales engineer today- we’ll be happy to help you out.