Troubleshooting Vertical Turbine Pumps

When an issue arises with a vertical turbine pump in a water well, contractors often have their own troubleshooting method. Some take a shotgun approach—attempting to resolve the issue through trial and error. Unfortunately, this can lead to unnecessary work, as well as wasted time and money.

A more careful approach often gets the job done. This consists of asking questions, surveying the water well/pumping system, and performing and analyzing a pumping test. The result will set the project up for a cost-effective and efficient resolution.

Getting Started

When a well owner states their flow rate or discharge pressure is not meeting expectations, this should be the start of a question-and-answer session that covers issues easily addressed on the surface without pulling the pump.

I begin by asking the following questions based on how familiar I am with the system. Based on the answers, a contractor can often develop an efficient resolution to solve the customer’s problem, saving time and money while also raising their own credibility with the customer.

1. Did the system ever meet expectations? – This question can uncover if the head conditions were properly calculated and determined in the first place.

2. Have head conditions changed on the surface? – Could changes have been made to the system? Changes such as the addition of treatment equipment or water-using components could require updated hydraulic calculations to determine changes in capacity or discharge pressure requirements.

3. Is the pump speed operating as expected? – This should include a review of a variable frequency drive if one is being used to ensure it is properly programmed to match the expected rotational speed of the pump.

4. Could there be issues with the discharge valves and piping? – A survey could uncover a partially closed valve in the discharge piping or a partially opened valve diverting flow.

5. Are the impellers set properly? – This could come into play when a vertical turbine pump with semi-open impellers is being used (e.g., raising the impellers by only .040 of an inch above the ideal position for a semi-open design will result in a decrease of up to 10% of flow at corresponding heads).

Diving Deeper

If the issue is not surface-related, then it’s time to perform a step drawdown pumping test. This allows you to operate the pump and water well under a controlled environment without pulling the pump.

The ability to accurately diagnose a system problem is only as good as the data collected. Before you begin testing, make sure to measure the right components with the right equipment.

1. Flow Rate – There are two main methods for measurement:

a. Orifice Tube – This is considered a simple, proven method. If constructed properly and measurements are taken carefully, then an orifice tube is extremely accurate. The major disadvantage is that water must be allowed to flow through the orifice and to waste.

b. Flow Meter – An assortment of flow meter types can be used, but limitations should be understood.

2. Water Levels – Employ a method of determining the water level:

a. Static Water Level (SWL) – This is the natural level of water in the well when it is not producing water. Care should be taken to ensure the SWL has recovered completely if the well was pumping prior to starting the test. This should be done by checking the water level with the pump off. Wait five minutes and check again if the static water level has risen. Wait an additional five minutes and check again. Continue until there is consistency in multiple water level checks.

b. Pumping Water Level (PWL) – Once a water well’s pump is turned on, it evacuates water from the casing. The lowering of the water level from the SWL creates a pressure differential causing water to flow into the well from the aquifer in its effort to raise the level back up to the SWL. The water level continues to lower until the rate of water flowing into the well equals the rate of water being pumped from the well. This new level is the PWL.

c. Methods of Measuring Water Levels – Portable water level indicators with a graduated electrical wire are battery operated and designed to be lowered between the well’s casing and the pump’s column pipe. These are simple to use—assuming the probe at the end of the wire can successfully be lowered to the water level.

Their accuracy can be decreased in wells with oil-lubricated pumping equipment as oil sitting on top of the water can tend to coat the probe’s contacts, interfering with their operation. Airlines are also a popular method; their accuracy is dependent on the pressure gauge used and knowing the full length of the airline.

3. Above-Ground Pressure Readings: An accurate pressure gauge must be used to enable the collection of discharge pressure near the discharge head. This must be positioned upstream from a gate valve to control the flow and wellhead discharge pressure.

4. Motor Amperage and Relative Solids Check:

a. Checking a motor amperage can provide valuable data, allowing operators to determine if the motor’s load is where expected.

b. Determining how much solids a well is producing and comparing it to past tests can be valuable in determining well issues and understanding why a pump’s wear may be progressing at an accelerated rate.

5. Column Friction Loss: This is the amount of head loss that is experienced by the flow of water through the column pipe. This amount cannot be measured in the field and must be calculated using knowledge of the column pipe and shaft size and length as well as the flow rate. It can be obtained using a column friction loss chart that is provided in the engineering manual supplied by most vertical turbine pump manufacturers.

6. Specific Capacity: This is defined as gallons per minute (gpm) divided by the drawdown of the water level. Drawdown is the difference between the pumping water level and static water level. It is essentially a measure of the ease that water can flow into the well.

For example, if a well has a static water level of 100 feet, a pumping water level of 150 feet, and a flow rate of 500 gpm, then the specific capacity is calculated as: 500 gpm/(150 feet – 100 feet) = 10 gpm/foot of drawdown.

Figure 1 is an example of a chart used to capture and present testing data. You can download a complimentary Step Drawdown Pump Test Form at franklinwater.com/pump-test.

The Step Drawdown Test

In an ideal situation, the discharge piping of a water well includes a tee with a gate valve following it. This allows isolation of the well from the system by closing it during the test. On the open end of the tee, a gate valve allows for flow and discharge pressure control. This provides a place to connect an orifice tube for flow measurement.

If there is no tee in the discharge piping, a decision must be made regarding whether to use an in-place flow meter, a portable flow meter, or (if a spool piece of the piping can be dropped) an elbow with a gate valve and orifice tube.

• Determining Test Points: Decide these beforehand. I like to set up the pumping test based on discharge pressures. This would include the system’s normal pressure (the high end of it for an interval) as well as testing one point above this, one point below, and another test point with the gate valve in the completely open position.
• Performing the Pump Test: Begin the test procedure by obtaining an accurate static water level reading, then starting the pumping equipment. Obtain a minimum of three readings five minutes apart for each test point. At each point ensure that the discharge pressure is still at the planned test point, adjusting the gate valve if necessary. Note the flow rate, the pumping water level, and the motor amperage (amp readings of all three legs for three-phase systems).
• Shut-Off Head Reading: I like to get one reading at shut-off head (readings with the gate valve completely closed). A cautionary note here: I only do this if I can calculate the expected pressure reading and ensure that no piping or valves will experience a pressure above the pressure rating.The valve should be shut for a minimum amount of time: just enough to note the pressure reading. Then open the valve up again to avoid any damage to the pump itself. The value of having a shut-off pressure is that this one data point takes out flow rate and pumping water level data from the equation as no water is flowing and the pump is lifting from the SWL, so there is no PWL data point to read.

 

 

Analyzing Step Drawdown Test Results

With your test performed, it’s time to analyze the results (Figure 2). There are two parts to this process:

1. Analyze the Pump’s Performance –This consists of plotting the pump’s performance onto its factory curve. The farther below the new curve is from the factory curve is an indication of pump wear and will determine how much flow/pressure decrease is due to pump wear.

• • The horizontal axis depicts the flow rate, usually in gallons per minute, and is read directly from the gpm column of the datasheet.
  • The vertical axis depicts the head generally measured in feet. This is read from the Total Dynamic Head column of the datasheet, which is a summation of the pumping water level, the above-ground pressure converted to head in feet, and the column friction loss.
  • Plot the points as well as the shut-off head point onto the curve. Connecting these points will create a curve that runs parallel to the pump’s factory curve.

2. Analyze the Well and Aquifer’s Performance – This should be a comparative analysis that looks at the current test results compared to past tests. You’ll specifically be looking for changes in SWL and specific capacity:

• • Are there changes in SWL? This will affect a pump’s performance as a pump must sacrifice flow for additional lift. SWL changes can occur for several reasons, including over-pumping/lack of recharge in the general area, interference from other nearby wells, or regional influences where aquifers are in direct hydraulic connection to a surface body of water (such as an alluvial aquifer whose water level rises and falls with the river water level).
  • Are there decreases in specific capacity as compared to past tests? This indicates that plugging is occurring at the aquifer/wellbore interface. Plugging can and often is caused by a combination of mineral encrustation, bacteria, and fine grains of the aquifer flowing from the aquifer into the gravel pack or well screen itself.

Delivering Solutions for the Job

When the pump’s performance is the cause of the degrading performance, it should be pulled, disassembled, and inspected. Pump repairs can often be performed with the aim of restoring the pump’s clearances to factory tolerances by adding/replacing wear rings, bowl bushings, pump shaft, and lock collets.

When the static water level is the cause of the issue, the solution is more challenging. Adding head to the hydraulic design of the pump paired with a possible lowering of the pumping equipment (assuming there is room) could provide a resolution.

When the specific capacity of a well drops below 25% of what it was originally, a rehabilitation procedure should be performed. A rehabilitation procedure designed to remove the plugging agents can often bring a well’s performance back up to peak performance.

A carefully performed and analyzed pumping test can pinpoint the cause of a water well system’s degradation and justify the considerable cost associated with maintenance. Care should be taken to prepare and present a well’s owner with an easy-to-understand pumping test report for their records. Then offer to perform a pumping test on a regular basis to ensure future problems are handled quickly. A pumping test can form the basis of a preventative maintenance program that helps to keep you close to your customers and their business.

As featured in Water Well Journal's article: Troubleshooting Vertical Turbine Pumps