Validating the Hysteresis of Pressure Switches on Pipeline Pigging Systems

Application

A pipeline integrity services company needed a reliable way to test the hysteresis of pressure switches used to control its pipeline pigging systems.

The company used a Druck DPI 802 Pressure Indicator/Loop Calibrator to improve the accuracy and consistency of its pressure switch testing process compared with previous methods involving a mechanical hand pump and analogue pressure gauge.

By accurately measuring the pressure switch's activation and reset points, technicians could identify potential switch problems before the pigging equipment entered a pipeline.

For Canadian oil and gas operators and pipeline maintenance teams, this type of verification can be important when preparing pipeline inspection equipment for operation.

Background

Pipeline integrity service providers help oil and gas companies evaluate the condition of their pipelines.

One method involves using pipeline inspection gauges, commonly known as pigs, which travel through the pipeline to perform cleaning, inspection, or other maintenance functions.

Inspection pigs equipped with appropriate sensors can collect information about the pipeline's condition, including potential cracks, corrosion, and mechanical damage.

Before a pigging system is deployed, technicians must confirm that its critical components are operating correctly.

An equipment failure during a pipeline inspection can disrupt operations and may require additional work to recover the pig before normal service can resume.

For this reason, the pipeline integrity services company sends a field technician to perform a comprehensive test and calibration check of the pigging system before it is used.

Why Pressure Switch Hysteresis Matters

An important component of the company's pigging system is a pressure switch used in its operating control arrangement.

The switch must respond correctly to changes in pressure for the system to operate as intended.

One way to verify its performance is by checking its hysteresis, which is the difference between the pressure at which the switch changes state and the pressure at which it resets as pressure moves in the opposite direction.

For example, a pressure switch may activate when pressure increases to a specified value and reset only after pressure falls to a lower value.

This difference helps prevent unnecessary switching when pressure fluctuates near the operating point.

However, unexpected changes in the activation or reset pressure may indicate wear, mechanical damage, or other problems within the pressure switch.

In the original application, technicians found that checking the switch's operating and reset points helped identify switches that might fail during operation.

Limitations of the Previous Testing Method

Previously, the company used a mechanical hand pump and pressure gauge to determine the pressure switch's trigger point.

The technician would manually apply pressure and observe the gauge reading when the switch changed state.

Although this method provided a basic indication of switch operation, it depended on the technician reading the mechanical gauge accurately at the correct moment.

Manual readings could produce inconsistent results, particularly when trying to identify small differences between the activation and reset pressures.

The company needed a more precise and repeatable way to measure these switching points.

Installation

To improve the testing process, the pipeline integrity services company incorporated a Druck DPI 802 Pressure Indicator/Loop Calibrator into its existing test box.

The test box contained the equipment required to check and calibrate the pigging system before deployment.

As part of the test procedure, the field technician connected the DPI 802 to the pressure switch and used a hand pump to apply pressure.

The calibrator's dedicated pressure switch test function allowed the technician to identify the pressure at which the switch activated and the pressure at which it reset.

In the original application, the DPI 802 was used to display switch points with a reported resolution of 0.01 psig, equivalent to approximately 0.07 kPa.

The relevant operating points were generally between 45 and 50 psig, approximately 310 to 345 kPa.

These values were specific to the equipment described in the case study and should not be treated as universal pressure switch settings.

If the measured switching points fell outside the acceptable operating range, technicians could investigate whether the switch had a damaged diaphragm or another fault.

Where a switch failed to meet the specified operating requirements, it could be replaced before the pigging system was put into service.

This approach provided a more reliable method of checking pressure switch operation than relying solely on manual readings from a mechanical gauge.

Advantages

The Druck DPI 802 includes a built-in software function specifically designed for pressure switch testing.

Once connected to the pressure switch, the instrument measures and displays the switching points as pressure is applied and released.

This provides several practical advantages over the previous testing method.

More Consistent Measurements

The digital display allows technicians to determine switching pressures without relying entirely on manual interpretation of an analogue gauge.

Improved Hysteresis Verification

By identifying both the activation and reset points, technicians can calculate the difference between them and compare the results with the switch's specified operating requirements.

Reduced Risk of Undetected Switch Problems

Identifying abnormal switching behaviour before deployment can help prevent a faulty pressure switch from being used in a pigging system.

More Efficient Field Testing

The built-in pressure switch test function simplifies the measurement process and reduces the need for repeated manual readings.

Reduced Operational Disruption

Verifying critical pressure switches before deployment helps reduce the risk of avoidable equipment failures during pipeline inspection operations.

For Canadian pipeline maintenance teams and field service contractors, these benefits are relevant when reliable equipment operation and accurate calibration records are essential.

Equipment

The original application used a Druck DPI 802 Pressure Indicator/Loop Calibrator with a measurement range of 0 to 20 bar, equivalent to approximately 0 to 2,000 kPa or 0 to 290 psi.

The DPI 802 is a handheld pressure measurement and loop calibration instrument used for pressure testing, switch verification, transmitter calibration, and loop diagnostics.

In this application, its pressure switch test function was used with an external hand pump to determine the actual switching and reset points.

When selecting a pressure calibrator for similar applications, technicians should confirm that its measurement range, accuracy, resolution, pressure connections, and calibration documentation are appropriate for the pressure switch being tested.

JM Test Systems supplies Druck instrumentation and calibration equipment for industrial measurement applications.

Canadian customers can also explore the company's instrument calibration and certification services, including pressure switch calibration and digital pressure calibrator services.

Practical Takeaway

Pressure switch testing is an important part of preparing pipeline pigging equipment for reliable operation.

A pressure switch that appears to function correctly during a basic check may still have activation or reset points outside its specified operating range.

The case study demonstrates how a digital pressure calibrator with a dedicated switch test function can help technicians identify these problems more consistently.

For Canadian oil and gas operators and pipeline integrity service providers, four considerations are particularly important:

1. Verify both switching points. Check the activation and reset pressures rather than confirming only that the switch changes state.

2. Compare results with the equipment specification. Acceptable switching pressures and hysteresis depend on the switch design and application.

3. Use a suitable calibrated reference instrument. Confirm that the pressure calibrator provides the required range, resolution, and measurement accuracy.

4. Investigate abnormal results before deployment. A switch operating outside its specified limits should be assessed and repaired or replaced as appropriate.

The Druck DPI 802 illustrates how specialized pressure calibration equipment can improve the consistency of field testing and help technicians make better decisions before critical equipment enters service.

For Canadian facilities, the objective is to identify potential failures before deployment rather than troubleshoot them after the pigging operation has begun.