Two Key Terms In Pump Specifications - GPM & Head
We live, eat, and breath pumps. We can have long discussions about atmospheric pressure at different altitudes and its effect on pumping operations. But, we know that most people, even those who use pumps every day, don’t know the terms and may find it hard to answer questions necessary to determine the right pump for their needs.
There are two terms that, when applied, can help you create a shortlist of pumps when looking for a water/fluid pump.
GPM and Pump HeadTypically, when looking for a water pump, people focus on two areas: How much or how fast can it pump, and how much pressure will it have. Like most industries, in the pump business, we have our own terms for these things.
Gallons Per Minute (GPM)When we talk about how much water a pump will move, the volume, we will refer to the Max Flow Rate as measured in Gallons Per Minute (GPM). But, this is not a static number. The GPM is affected by how high the water needs to be pumped… the Head.
Feet of HeadThe term Head is related to the forces of gravity on the vertical flow and the PSI needed to “push” the fluid against gravity, but we won’t go into a deep explanation here. In short, it deals with how high the fluid can be pushed through the system. There are three types of Head measurements.
- Total Head: The height from the original water source up to the highest point to which the fluid will be pumped - the discharge point.
- Total Suction Head: The height between the fluid source (what is being pumped) and the pump itself.
- Total Discharge Head: The height from the pump itself and the highest point that the fluid will be pumped in the system.
Maximum Suction LiftAffecting the Total Suction Head is the Maximum Suction Lift, which is the height from which a pump can “pull” water. This number is impacted by atmospheric pressure and friction in the system. (The terms “pull” or “suck” are misleading. The atmospheric pressure actually “pushes” the fluid into the vacuum. But, that’s a whole other post to come.) Maximum Suction Lift affects the Total Suction Head, and thus the Total Head.
GPM at Feet of HeadNow, we combine GPM and Head to understand how a pump meets some of the performance requirements that your applications have.
Pumps have measurements of “GPM at X feet of Head”. Given the same power and pump design, there is a trade off between Head height and Gallons Per Minute.
Typically you will see pump ratings like this:
140 GPM at 40 Feet of Head
100 GPM at 60 Feet of Head
60 GPM at 70 Feet of Head
The actual ratings will vary by pump.
So, for the hypothetical pump above, it will work if you need to pump the water to a height of roughly 40 feet, at a dewatering at a rate of 140 gallons per minute. If that is not fast enough, can you lower the discharge point, or find another pump that can meet the specs?
You may also see these terms prepended with “Static” or “Dynamic”. This is related to the impact of friction within the system. As an example, fluid flowing through a hose has friction where it touches the interior of the hose. This inhibits flow and must be taken into account when determining the Total Head. Don’t worry about this, as the pump specs take these things into account.
Other sources of friction can come from the liquid itself. Highly viscous fluids or water with a lot of suspended solids and debris will create more friction and degrade the performance.
Affects Altitude PumpsAs mentioned above, pumps don’t suck water as much as the atmospheric pressure pushes the water into the vacuum created in the pump. The higher you go, the less atmosphere there is above you, thus the less atmospheric pressure there is to “push” the water into the vacuum. This reduces the Total Suction Head.
Additionally, in higher altitudes, the air is thinner. This provides less oxygen for gas and diesel engine combustion, thereby reducing the HP of the engines. This impacts the Total Discharge Head.
When discussing the types of pumps and requirements, be sure to keep the operating altitude in mind. The experts at PumpBiz can assist you in this regard.
Answering how much and how far (high) you have to pump the fluid is one of the first steps in determining the right pump for your application. There are many other factors, such as the type of fluid being pumped, the environment in which it operates, or the frequency of operation. Many pumps are designed for applications such as trash pumps, irrigation, fire, etc. Once you know the performance requirements, a good next step is to look under applications to see the range of pumps that are designed for those needs. If you are unsure, please contact us. We are always happy to help.