Diagnosing Common Issues with Variable Displacement Piston Pumps

Variable displacement piston pumps, particularly those made by Rexroth (such as A2VK, A7VK, and A2FK) and Rotary Power (C01, C04, C07, C20), are essential for many hydraulic and chemical pumping systems. Known for their efficiency and flexibility, these pumps can encounter issues that affect performance. This post explores how to diagnose common problems, such as loud noises, pressure buildup, and inadequate flow rates. With a better understanding of these challenges, you can keep your pump and system operating at peak efficiency.

Common Problems with Variable Displacement Piston Pumps

Loud Noises from the Pump

Hearing unusual sounds from your chemical pump can signal trouble. These noises, often described as grinding or rattling, can indicate serious issues.

Bearing Failure is often the main source of loud noises. Over time, these components wear out, particularly if the pump has been in service for many years. For instance, a pump that has operated for over 8,000 hours may experience bearing failure, which can lead to performance issues.

Magnetic drive systems can also contribute to noise. If components within the drive are worn or damaged, they may create noises that sound like they are coming from the pump. It is important to inspect the mag drive when diagnosing noises in the pump area.

It is possible to continue operating the pump, if it achieves desired flow rates and pressures. However, the pump should be replaced at the next earliest convenience to prevent unplanned downtime.

Inability to Build Pressure/Flow

If your pump fails to build pressure, it is crucial to identify the root cause. Pressure issues can severely disrupt the efficiency of the entire system.

1) If the pump is new and fails to build pressure/flow "out of the box" there could be several contributing factors:

The plumbing may not be connected properly.

When replacing a pump it is good to verify the correct opertaing mode. A Closed Circuit pump will not operate properly when connected to plumbing like an Open Circuit Pump, and vice versa. You can refer to the way the pump was plumbed into the system to determinine the operating mode, or you can refer to the pump model code.

(For Rexroth Open Circuit pumps, the supply plumbing must be connected at the back or top of the pump. For Closed Circuit, the supply line must be connected to the bottom pressure flange)

A2VK Operating Modes

The Rotation of the pump may be incorrect

Pumps are set up to operate in either a Clockwise or Counterclockwise rotation. Running a pump "backwards" may yield some flow or pressure, but will certainly not provide the rated flow, pressure, or service life.

The best way to determine the proper rotation is to refer to the pump model code.

For example, for a clockwise rotation, the pump shaft must rotate clockwise when looking at the pump shaft, from the front of the pump.

In addition, it would be wise to verify which direction the motors are actually turning in. Changes in power supply wiring can alter the direction of the motors, such as when a machine is moved.

The Pressure Relief Valve may not be set correctly:

The pressure relief valve is meant to dump excess pressure back to the pump case. However, these valves can be set manually. The pump will never achieve its desired pressure if the relief valve is set to relieve pressure at a pressure lower than the desired pressure at the mix head.

The Pump Size may be inadequate:

As a gut check its always good practice to compare your desired flow rate (grams/sec) to the rated flow rate of the pump in question. Operating above or below the rated flow rates will yield inconsistent or inadequate pressures and flow rates. You can refer to our Pump Page for specifications on Rexroth and Rotary Power Pumps.

2) If the pump has been in service for awhile:

There may be wear on the rotary group components:

Wear in these parts can reduce pressure and overall performance. This often happens in conjunction with bearing failure. The longer the pump operates, the faster the components wear.

The Vernier Flow Control may be need adjustment:

For variable displacement piston pumps, the amount of volume per revolution can be adjusted by the Vernier, or control handle, on the pump. The control handle may have been inadvertently moved to an incorrect setting, leading to a lower or higher output than desired.

This is especially important to consider when the PLC Control system utilizes VFD Frequency Drives to control the electric motor speed based on the desired output, as measured by the Flow Meter. The PLC may not be able to compensate for if the Vernier adjustment is set too low or too high.

Other Common Issues: Leaks

Leaks can pose a major challenge for variable displacement piston pumps, originating from various sources.

Improper fittings are a typical cause of leaks. If connections are loose or incompatible, hydraulic fluid can escape, compromising system efficiency.

For reference, Rexroth Pumps utilize a Flat Face Port with Metric Threads on the pressure flange. Using an improper thread (such as SAE or BSPP) would yield insufficient sealing.

O-rings and gaskets are also critical. Damage or absence of these small components can lead to significant fluid loss. Regular inspections of all fittings and seals can prevent leaks and ensure operational integrity.

At MHR, it is not uncommon for us to find improperly sized fittings or Orings installed on pumps that have been leaking.

A Word on "Best Practice"

One very important factor to consider for your plant operation is the cost of downtime. Can you afford to be without a pump for a few days? What is the true cost of the downtime, for you and for your customer?

It is always best practice to keep a spare pump on hand at all times. At Mix Head Repair, it is not uncommon for us to have customers calling with an emergency need for a pump. While we do our best to keep replacement pumps stocked, it can still take a day or two for pumps to be shipped to your facility.

In the case of Rexroth pumps, it is also worth considering keeping a spare pump for each handwheel orientation type you have.

Mix Head Repair keeps Rotary Power and Rexroth pumps in stock for the most common models. Other models may be available to be expedited, but being proactive and keeping a spare on hand guarantees you can be up and running again within hours, not days.

It may be worth looking online for a "Downtime Cost Calculator" to understand the true cost of downtime for your particular situation.

How MHR Can Help

At MHR, we recognize the complexities associated with maintaining rotary piston pumps. We are equipped to diagnose and repair what you've encountered.

Whether you are dealing with loud noises, pressure buildup, flow rate developments, or leaks, our experienced technicians can identify and resolve the issues effectively.

We also offer replacement parts, fittings, seals, and gaskets essential for keeping your pump in prime condition. Our goal is to serve as your one-stop shop for all chemical pump needs, ensuring your systems operate reliably and efficiently.

Need a quote on a new or repaired pump?