TROUBLE SHOOTING TIPS

 

LOW PRESSURE

DISCUSSION

Explanation

        This document covers both general complaints of low pressure and also the LOW PRESSURE  safety feature of a pump station (if so equipped).  The principles of the LOW PRESSURE  safety system are explained first, then the general principles of low pressure problems are explored.  Words in pink are comments on other brands of pump stations.

 

The Purpose of the Low Pressure Alarm

    The Low Pressure safety system protects the property owner by shutting down the pump station if a mainline fails or if valves remain constantly on.  Irrigation Craft pump stations cannot be harmed by low pressure because all our pumps are non-overloading.

    Other manufacturer's pump stations may be harmed by low pressure caused by high flow rates because the pumps may be capable of overloading the motors.  If high flow rates cause motor heating and the overload system is not accurate or working properly then the pump motor may burn out under excessive high flow conditions.

 

Low Pressure Feature on Pump Stations

  On Irrigation Craft pump stations the LOW PRESSURE feature works as follows:

When mainline pressure drops below an adjustable set point continuously for 3 minutes the station will be retired and the LOW PRESSURE alarm light on the panel door will be turned on.  The station will not restart until the RESET pushbutton is depressed.

The low pressure adjustment switch is a dedicated pressure switch located inside the control panel.  Based on experience, we find this switch to be stable and reliable almost to the extreme, and therefore not likely to fail or change adjustment.

Low pressure indicates either excessive high flow or a problem in the pump area.

On other brands of pump stations the time delay period may be different.

On other brands of pump stations there is seldom a dedicated pressure switch for the low pressure feature.  Often the same pressure switch is used for both pump start and low pressure.  This presents a complication to the service technician who must remember that any adjustment to this switch affects both low pressure and start pressure simultaneously.  This lack of a dedicated low pressure switch may prevent the technician from making a satisfactory adjustment that works equally well for start pressure and low pressure shut down.  As a result the final setting may be harmful to the irrigation mainline by causing excessive pressure cycling.

 

Diagnosis

        Your first goal will be to determine if the problem is in the field, or at the pump area.  Field problems would be EXCESSIVE DEMAND, pump station problems would be INSUFFICIENT SUPPLY.  Excessive Demand is when the demand exceeds the designed capacity of the pump station.  Insufficient Supply is when the pump station is unable to provide the flow required by design.

 

Check the Obvious First

        Use the diagnostic method called “divide and conquer”.  There is one key diagnostic observation that may enable you to quickly focus on the field, or on the pump station.  The key question to be answered is: how much water is being pumped?  There are several ways to determine this.

        If there is a reliable flow meter, just read the flow rate.  If the flow rate is much higher than it should be you have a field problem, start walking.

        If there is no flow meter use an amp meter and find out from the motor nameplate what the motor FLA (full load amps) is.  The rule is, the more water that flows, the more electrons will flow.  Electrons are counted in amperes (amps).  Measure the amps drawn by the motor under the low pressure operation conditions.  If the amps are close to or exceed FLA, then the pump is pushing a lot of water and pulling a lot of electrons through.  Start walking, you have a field problem.  If the amps are low, then you are not moving much water and your problem is in the pump area.

        That simple little exercise above is good, but not always possible.  You may have a moderate low pressure problem where there is no obvious huge clue enabling you to quickly determine the direction to look in.  However it is important that you look at flow rate or amps first because you look like a fool checking out the pump station when half a mile away there is 2,000 gpm flowing into a vacant lot.

More Accurate Diagnosis

Use the diagnostic that best suits your system.

1.     RATE OF PRESSURE DROP - Turn off all known flows, turn the pumps on manually and pressurize the mainline to maximum pump pressure, then turn the pump off.  Does the pressure drop and how fast does the pressure drop?  If the pressure drops rapidly then you may have a large mainline leak or valve open.  If the pressure drops slowly then the leak may be too small to cause the low pressure problem.  On very large or long mainlines this “rate of pressure drop” method may not be valid because the mainline stores large amounts of energy and rate of pressure drop is difficult to judge.

2.     FLOW METER - If there is a flow meter read the flow rate and pressure at the pump discharge (we will presume there is a pressure gauge).  Then use the pump curve to determine if the pump is delivering the appropriate amount of pressure at the indicated flow rate.  Remember to subtract from the pump curve the following items when making this judgment: suction lift, suction losses, and manifold losses.  If the pump is delivering approximately the correct amount of pressure at the measured flow rate according to the pump curve, start walking, the problem is definitely in the field, demand is excessive.

3.     CURRENT DRAW - If there is no flow meter then obtain a “feel” for how much water is being pumped.  Use an ammeter to check the amount of current being used by the pump motor.  Current consumed by the pump is an excellent flow indicator.  If the motor is drawing close to or above the FLA of the motor, then the motor is working hard to supply a high flow rate, indicating a field problem.  Also remember this one simple and reliable fact, ALL HYDRAULIC PROBLEMS ON PUMP INTAKES (YES, EVERY LAST ONE) RESULT IN LOW CURRENT DRAW.  If an intake screen is clogged, if there is an air leak, if the impeller is clogged, or even if the intake line is completely capped off, current consumption will be low, ALL INTAKE PROBLEMS CAUSE LOW CURRENT DRAW (except mechanical rubbing of the spinning impeller against the volute.

4.     Once the source of the problem is determined (PUMP or EXCESSIVE DEMAND), use the appropriate check list below to find the problem.

 

FIELD PROBLEMS THAT CAUSE LOW PRESSURE

1.     MODERATE TO LARGE MAINLINE LEAK.

2.     LARGE LATERAL LEAK.  Lateral lines are all pipes downstream of the zone valve.

3.     ZONE VALVE STUCK ON.

4.     SLOW CLOSING VALVES - Prove there are no slow closing or “sticking” valves with the following procedure:

Turn each zone on, then advance to the next zone.  Be careful to observe that each zone turns off within 30 seconds after it is turned off.  If a valve takes longer than 30 seconds to shut down then that valve may be suspected of having problems.  Experienced technicians can watch the pressure gauge and “see” the valves opening and closing by the behavior of the pressure gauge as they move through the zones on the controller.

5.     CONTROLLER SEQUENCING - Excessive demand caused by controller overlap can cause low pressure events.  Examine all controllers to determine if any controllers are operating when they should not be.   A pressure recorder can be used in finding this problem by revealing at what time of the day or night the high demand is occurring, and then comparing that time with controller timings.

6.     LOCAL PROBLEMS - Compare zones of similar size.  If two zones are equal in size and yet one has low pressure then check the following: pipe sizing problem, distance problem (is one zone very much further away from the pump?), or a lateral pipe problem.  If lateral pipe sizes are too small low pressure can result in that zone, while similar zones work fine.  Another local problem is incorrect nozzle sizing, which is discussed directly below.

7.     INCORRECT NOZZLE SIZING –  Check to see that head nozzle sizes are correct.  Installation of just one size larger can cause severe low pressure problems.  During maintenance of systems the technicians often do not pay attention to replacing heads with new heads having the same nozzle sizes.  Doubling the size of nozzles in a zone doubles the flow rate of that zone.  Another nozzle problem involves low angle nozzles versus normal nozzles.  If a zone is designed or installed with #8 nozzles, but the contractor or maintenance people install #15 nozzles the flow rate is tripled.  #15 nozzles have 3 times the flow rate of #8 nozzles.

 

PUMP PROBLEMS THAT CAUSE LOW PRESSURE

1.     ROTATION – On three phase systems the pump can rotate in either direction.  If rotation is incorrect, pressure will be close to but slightly below normal.  If the pump is rotating incorrectly the pump will exhibit more noise than usual.  If rotation is incorrect, switch any two wires at the bottom of the motor starter feeding power to the pump and the rotation will be reversed.

2.     DISCHARGE or ISOLATION VALVE - Closed or partially closed station discharge valve.

3.     DUPLEX PUMPS – Is the lag pump starting and retiring properly? 

4.     PRESSURE CONTROL VALVE - Is the valve working and adjusted properly?  Is there pressure available on the high side of the control valve that can be delivered to the mainline?

5.     RESTRICTED INTAKES and TIGHT WELLS - Symptoms will be an abnormally high vacuum reading and pump cavitation.  Clogged intake screens, clogged or malfunctioning check valves can cause low pressure.  A well is said to be “tight” when it resists the flow of water at the flow rate required. A key diagnostic observation would be correct operation of the pump and system at low flow rates, but as the flow rate increases the well resists the flow sufficiently to cause pump cavitation.  A tight well will have exactly the same features as partially clogged intake screens or malfunctioning check valve.

6.     CLOGGED IMPELLER - There is no easy way to prove or disprove this.  A high vacuum reading at high flow rates is one indicator that the impeller is NOT clogged with debris.  The most obvious method is to split the pump case apart and visually inspect the impeller.  Is there debris in the eye of the impeller?  Pass a wire down through each vane from the outside edge all the way to the eye of the impeller to check for small rocks that can get stuck in the pump vanes and be hard to see.  Perform this inspection carefully because many people have missed objects stuck in the middle of the vanes where they cannot be seen.

7.     NPSH PROBLEMS – NPSH stands for “net positive suction head”.  NPSHR stands for NPSH REQUIRED, and NPSHA stands for NPSH AVAILABLE.  NPSHA must be equal to or greater than NPSHR, or the pump will cavitate and low pressure may result.  Pumps require a minimum amount of pressure at their inlet or they will cavitate, causing them to fall short of their rated performance.  On pumps above the water source the lift may be excessive.  One misconception prevalent in the industry is that submersible pumps and pumps with flooded suctions cannot have problems in this area.  Any pump can cavitate and there is no way of knowing exactly what is happening unless the NPSH calculations are done.

8.     INTAKE LINE AIR LOCK – When intake lines are not graded up towards the pump, air locks develop that restrict water flow in the intake line.  Air locks will usually clear up by operating the system, but in severe cases they may not clear up.  Water flow is restricted causing high vacuum readings and pump cavitation.

9.     INCORRECT CHECK VALVE - A common problem on larger pumps (above 3 horsepower) is an original design or installation error.  Large pumps require high flow check valves, which are expensive and sometimes difficult to obtain.  Symptoms will be high vacuum reading and pump cavitation.  This problem is very prevalent in the industry, which is why Irrigation Craft supplies the correct check valves with our pump stations.

10.   SMALL INTAKE LEAK – Unlikely and rare, a small leak in the pump intake line can cause low pressure.  Remember that large intake leaks cause loss of prime, not low pressure.  Small air leaks will cause noisy pump operation as the air moves through the pump, and sometimes there will be surges of pressure and flow as air moves through the pump.

 

Learn more about Intake Lines

 Learn about Air Locks

 

Overload No-Flow Rapid Cycle Air Lock No Start Loss of Prime Volute Low Pressure High Pressure

 

 

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