In-line testing of pressure relief valves (PRVs) is gaining acceptance in the industry and is increasingly being integrated into plant process management programs. In-line testing is an attractive option for plant operators for several reasons. The plant remains in operation during testing, there is minimal disruption to plant personnel and daily routine, and maintenance costs are reduced since manpower and equipment aren’t needed to physically remove valves for offsite testing.
The main purpose of in-line testing is to verify the name plate setpoint of the valve. The setpoint is the inlet pressure under the valve seat at which the valve will vent to atmosphere. If the setpoint initially tests outside the test criteria, the valve technician can adjust and repeat the test until the criteria is met.
There are several in-line test systems available on the market today. Design and implementation details vary, but principles of operation and general purpose are common to all. This article provides a general understanding of the principle of operation and the common elements of in-line PRV test equipment.
PRINCIPLE OF OPERATION
In-line testers are designed to apply a lifting force to the valve stem until the valve begins to vent. Because of this, an in-line test system is often referred to as an auxiliary lift device (ALD).
With the known values of valve seat area and inlet line pressure, and the measured lifting force supplied by the ALD, the setpoint is accurately calculated. The valve technician can then adjust the valve as necessary to match the name plate set pressure. The following general formula is used to calculate the set pressure:
Force (lbs.) = [Valve Set Pressure (psi) – Line Pressure (psi)] x Valve Seat Area (in2)
COMMON ALD EQUIPMENT
While design details vary, ALD systems share several core elements that are fundamental for conducting a proper valve test. These include a load rig, lift mechanism, cable harness, feedback sensor, control panel and software. Portability is an important design goal of all ALD systems for the ability to maneuver in the plant environment and move quickly from valve to valve to complete the job.
The load rig is a mechanical structure that mounts on top of the valve and provides the support for the forces that will be exerted to lift the valve stem. To mount the load rig, the valve cap is removed, the mechanism is placed over the valve stem and is secured in place. The important design considerations are the maximum lifting force supported, overall height for mounting clearance above the valve, and the range of valve sizes the rig can accommodate.
The workhorse of the ALD is the lift mechanism; it applies the upward force on the valve stem and lifts the seat. The two main lifting methods used in ALD systems today are hydraulic or electric motor, both of which have their merits.
In a hydraulic design, a cylinder on the load rig produces a lifting force from pressurized hydraulic fluid in a hose. The pressure is typically applied with a hand pump by the technician increasing the fluid pressure in the hose, which in turn applies the lifting force.
In the second method, an electric motor is mounted on the load rig and a current is supplied, causing the motor to turn and apply the lifting force.
In both methods, the lift mechanism is mechanically linked to the valve stem through a spindle adaptor threaded onto the stem. Most ALD systems come with a set of spindle adaptors to accommodate a wide range of valve sizes.
For operation of the ALD and for the setpoint calculation, the amount of lifting force must be known. A load cell is an integral part of the lift mechanism and provides feedback on the load applied to accomplish the lift. At the point the valve vents, the measurement from the load cell is captured and used in the setpoint calculation.
The inlet line pressure is also a required value for the setpoint calculation. Often the line pressure is controlled to a determined setting by the plant control room and this value is used in the calculation. However, in situations where the line pressure is not known, a pressure transducer must be mounted in the line to provide direct feedback to the ALD system.
The cable harness is the bundle of cables that carry sensor signals, control lines and power off the load rig to a control panel where the technician will conduct the valve testing. A harness is typically in the range of 15 ft or greater and is covered in a heat-resistant sleeve to protect against damage if it comes into contact with steam and hot surfaces.
The nerve center of an ALD is the control panel. Signals from the sensors and lift mechanism connect through the cable harness to the control panel, which houses the electronics for signal processing, battery management and manual control of the lift mechanism. The technician conducts the test from the control panel to monitor progress, handles error conditions that may arise and verifies test completion.
Modern ALD systems come with application software providing features to help manage and control the test session. The software guides the technician through the test sequence, alerts to error conditions, displays graphical results and generates reports. A database of valve manufacturers, models and critical dimensions is usually included for ease of test setup. The application software runs on a computer that may be imbedded in the control panel or on a laptop computer. Functionality of the software can vary greatly from a basic manual test to fully automated process with setpoint calculation.
In-line testing of pressure relief valves is a growing trend. Testing with an ALD presents a cost-effective maintenance alternative for plant operators and a growth opportunity for valve service companies. This article has presented a basic understanding and common features of ALD devices and their operation. When it comes time to invest in a system, take care to understand the distinct features and advantages of each system to meet your requirements.