11192018Mon
Last updateMon, 19 Nov 2018 5pm

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Technical

Small Scale LNG Offers Opportunities for Valve Manufacturers

There are many areas throughout North and South America where there are pockets of “stranded gas,” and many communities that need that gas but have no pipeline infrastructure. That includes individuals and businesses like mines and manufacturers.

To respond to the needs of gas producers and their customers, Okra Energy has developed small-scale LNG plants that make it possible to get gas where it’s needed. They call it a “virtual natural gas pipeline,” and the core of it is modular liquefaction and regasification.


Preparing for a Plant Outage

Preparing for a Plant OutageFrom time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who might have missed them will be able to catch up on the best of the best. This article, “Preparing for a Plant Outage” initially ran on April 27, 2015.

It is well known that effective planning can save time and curtail expenses when production and utility plant areas prepare for upcoming maintenance turnarounds, outages, and shut-downs. Such planning is also a key component to effective management of both the internal maintenance employees and externally contracted technicians that perform the work. Since manpower resources make up a large portion of the total expenses associated with any outage, organizing those resources is as critical as organizing the process itself.

In many cases the outage is part of an existing plant preventive maintenance (PM) program: critical valves within the plant’s valve population have already been identified or repairs have already been scheduled. That PM program may depend on use of an existing computerized maintenance management system (CMMS), which can help when scheduling work to be performed and when reconciling a list of poorly performing valves already identified.

Bolting and the Space Shuttle Challenger Disaster

On Monday, January 27, 1986, the crew of the space shuttle Challenger was ready for launch and carefully loaded to the top of the multi-billion-dollar spacecraft. All signs were “go for launch” on that warm day at Cape Canaveral, FL.

As the technicians were closing the door of the cockpit and rotating the handle 90 degrees, a problem arose. They could not remove the handle from the door. No amount of pulling and pushing could unstick the handle, so Lockheed space operations engineers requested power tools to help in the removal.

As the world watched, the pad technicians located battery-operated drills and cutting blades, but when they got to the door, the batteries were low and could not remedy the problem. The issue was a growing embarrassment and taking up valuable time; the simple inexpensive bolt that holds the handle onto the shuttle had seized and was delaying the launch of the seven astronauts into space.

Creep Strength Enhanced Ferritic Materials in Thermal Power Applications

The designers of modern thermal power plants continue to work hard to improve plant efficiency by increasing the heat rate as a function of main-steam pressure and temperature.

As competition with wind and solar platforms continues to intensify, so does the need to eke out the maximum Mw output possible in a coal or combined cycle generating unit. For nearly three decades, the application of creep strength enhanced ferritic (CSEF) materials (e.g. P 91, P92, P911, P122, Gr 23, etc.) has been employed to lessen plant construction costs, while maintaining/improving piping system performance.

In-Line Test Systems for PRVs: Purpose, Principle and Equipment

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.

LOAD RIG

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.

Load Rig Components

LIFT MECHANISM

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.

SENSORS

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.

CABLE HARNESS

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.

CONTROL PANEL

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.

ALD SYSTEM COMPONENTS

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.

SUMMARY

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.


This email address is being protected from spambots. You need JavaScript enabled to view it. is vice president of sales and service at AccuTEST Systems, Inc.

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