One of the main issues that can cause pump problems is when your sprayer pump is run dry. Replacing or repairing a sprayer pump can be costly and time-consuming. It is important to prevent your pump from running dry, but what exactly does that mean to run a pump dry anyway?
Running a sprayer pump dry means operating the pump without any liquid in it. This can be damaging to sprayer pumps, especially centrifugal pumps. With no liquid to lubricate and cool the internal components of the pump, things will heat up and the pump can fail.
There are several factors that can cause a pump to run dry. The type of pump, the plumbing, the liquid being pumped, etc. In this article, we will look at how running dry will affect different types of sprayer pumps, as well as what causes it, and how to prevent it.
What Happens When Different Types of Sprayer Pumps Run Dry
The most common types of sprayer pumps are centrifugal pumps, roller pumps, and diaphragm pumps. None of these pumps should be run dry, however running them without liquid will have different effects.
Centrifugal Pumps
Running a centrifugal pump dry can cause significant damage to the mechanical seal. A mechanical seal is a device that is used to provide a seal around the rotating shaft of the pump. It is designed to prevent the liquid being pumped from leaking out of the pump as the shaft turns.
When there is no liquid passing through the pump, the seal faces are not being cooled and lubricated. They will begin to heat up due to the friction of the rotating seal face against the stationary seal face. This can cause the seal to wear out quite quickly and even cause the seal to fail. A seal failure from running dry will typically result in a substantial leak around the shaft.
Not getting enough liquid to the pump can result in cavitation. This is a phenomenon that occurs inside the pump when the liquid turns into vapor and the vapor bubbles burst. In addition to seal failure, this can severely damage the inside housing, volute, and impeller.
RELATED: Troubleshooting a Centrifugal Sprayer Pump
Roller Pumps
When a roller pump is run dry, the rollers and rotor are not lubricated, causing friction which results in excessive heat. This can cause the rollers to wear down quickly, and can also damage the bearings and seals in the pump.
Diaphragm Pumps
Although medium and high-pressure diaphragm pumps are lubricated, if run dry, it can cause damage to the diaphragms.
What Causes a Pump to Run Dry?
A pump running dry is due to a lack of liquid supplied to the pump. This seems like a straightforward issue but there are many potential causes that can lead to a pump running dry.
The Suction Hose is too Small
If the hose or pipe that supplies liquid to the pump is undersized the pump may be “starved” and run dry. The size hose or pipe used from the tank to the pump inlet should have an inside diameter no smaller than the inlet of the pump.
Suction Hose Collapsed
Pumps will create suction when they are in use. If the hose used on the suction side of the pump is not a reinforced hose rated for suction, it can collapse under the vacuum.
Pump Not Primed
If the inlet of your pump is not gravity fed from the sprayer tank then it is imperative that your pump is properly priming. Roller pumps and diaphragm pumps are self-priming. If you have an issue with one of these types of pumps not priming, then you’ll have to look for a system problem. This can be a result of an air leak in the suction side or a clogged strainer. You can find more specific details in this article on diaphragm pump troubleshooting.
Centrifugal pumps must be gravity fed unless they are self-priming. Gravity-fed or “flooded suction” means that the pump inlet is lower than the liquid level in the sprayer tank at all times. This ensures that liquid is flowing to the pump Inlet. If you have a self-priming pump, then the pump will draw liquid from a level below the inlet of the pump. It is essential to remember that a self-priming pump still needs to be primed when it is initially installed. You should also check before each use that there is liquid in the pump and it is still maintaining its prime.
Low Liquid Level in The tank
When you are operating a sprayer eventually the liquid level in the tank will decrease. As the liquid level decreases, the pump may be sucking air as well as liquid. This is a common cause for seal failure because It appears that there is still plenty of liquid in the tank, however, the liquid level is misleading. Although you see liquid in the tank, the suction from the pump can create a vortex in the bottom of the tank that draws air into the pump. This will lead to inadequate liquid in the pump and possible cavitation.
Leaks
This is less common but if there are major leaks in the supply lines obviously it could limit the flow to the pump. Like with a vortex in the tank or collapsed suction line, limited flow to the pump can result in cavitation.
How Long Can a Pump Run Dry Before Damage?
A mechanical seal can be damaged relatively quickly if a centrifugal pump runs dry. The exact amount of time it takes for a pump to be damaged will depend on a number of factors, such as the type of seal being used, the speed at which the pump is running, and the type of fluid being pumped. The primary concern in a centrifugal pump is the seal. In general, a mechanical seal can be damaged in a few minutes but in some cases, it may be less than a minute if a centrifugal pump runs dry.
How do I know if My Pump is Running Dry?
There are several signs that can indicate that a pump is running dry:
Noise
While in operation a centrifugal pump will make noise. It is generally a steady sound unless there is a change in the rpm or something else. When air passes through your centrifugal sprayer pump, or liquid stops moving through the pump at all, you will notice a definite change in sound. Turbulence in your water tank, especially around the outlet of the tank can induce air into the suction line. If enough air is in the suction line, you can hear a slight rumbling sound. Large pockets of air can result in a surge from the pump as the gulp of air passes through the volute, especially in gas engine drive pumps.
A vortex can also cause a pump to run dry. A pronounced vortex in the tank can produce a sucking sound from the pump. These sounds can be difficult to hear unless you know to listen and are standing close to the sprayer.
Seal Failure
If you find yourself replacing pumps or pump seals often, it may be that your pump is running dry at times and you do not notice. This can be frustrating, but if you are able to figure out the cause you can extend the life of your pump.
Ensure that your plumbing is not creating any potential issues. Consider suction hose size, clogged strainers, and even the outlet fitting in the tank. If you have recirculation in the tank, do not point the recirculation nozzles near the tank outlet. This can introduce turbulence and air into the suction line, potentially leading to a loss of prime and a run-dry scenario.
If you are interested in more information, this article on troubleshooting centrifugal pumps provides a more detailed guide.
Empty Tank
If you wait until you don’t have any liquid coming out of your spray nozzles, before shutting down your pump, it is already running dry. Even the small amount of time between when you notice you are out of liquid to the time you shut down your sprayer can hurt the seal and eventually will start to leak.
It’s important to keep an eye on the pump’s performance, and in case of any of these signs, it’s best to shut off the pump and check the inlet and discharge lines, the suction and discharge pressure, and if necessary, the pump’s internal components.
How to Keep Pump from Running Dry?
There are two different types of centrifugal pumps, self-priming and straight. Both of these types of pumps can run dry if not properly installed and operated. There are several ways to keep a pump from running dry:
- Ensure the entire fluid path from the sprayer tank to the pump inlet is now smaller than the inlet of the pump. This means that for a 2-inch pump inlet the tank opening, hose, strainer, ball valves, or any other plumbing fitting used needs to have an inside diameter that measures two inches or larger.
- Use a vacuum-rated suction hose on the inlet side of the pump. A true suction hose will be reinforced with a plastic or wire helix in the wall of the hose that gives it support and prevents it from collapsing.
- Do not install agitation nozzles into the tank near the withdrawal port of the tank. The turbulence from the agitation can result in air getting into the suction line.
- Use an antivortex fitting in the withdrawal opening in the tank. This type of fitting Helps Prevent A vortex from forming with the tank level gets low which can result in air getting into the suction line. Banjo Corp. makes 2 and 3-inch poly anti-vortex fitting that works great, you can find them online from Tank Depot.
- Always check that Any ball valves in the suction line that supplies the pump are open before starting the pump.
- Use caution when spraying on an incline. All of their baby enough liquid in the tank to properly feed the pump on level ground, but when on an incline the liquid moves to one side of the tank and away from the outlet that feeds the pump.
By implementing these methods, you can prevent your pump from running dry and protect it from damage, prolonging its lifespan.
