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Operator safety is one of the most important aspects to be considered for any system design. Sampling systems often handle samples at very high hydraulic pressures and temperatures. In some cases the samples can be hazardous and toxic gases. The system needs to be isolated and locked out (tag out) before inspection, maintenance, cleaning or repair. Isolation procedures must be clearly detailed and documented for any sample handling system.

Isolation of the system from any hazard to humans is the first step towards safety. The aim of an isolation procedure is to:

Isolate all forms of potentially hazardous energy (high pressure and temperature samples) to ensure that an accidental release does not occur

Ensure that entry to a restricted area is tightly controlled


Importance of Isolation Valves for High pressure and Temperature Steam Samples

Steam and water analysis systems (SWAS) are used for online monitoring of steam/ water samples from boilers and turbines. These samples are at very high pressure and temperatures.

In next generation ultra-supercritical power boilers, feed water pressures can be as high as 350 bar g and temperatures of superheated steam closed to 600 Deg C. These samples hold very high energy in terms of hydraulic pressure and temperature. Any exposure to these samples can be hazardous to humans. Also, during startups, samples need to blow down to ensure only clean samples are taken and clogging of sample lines is avoided. These valves are the first active component of the system that are directly exposed to high pressure and temperature samples.

Important aspects of Valve Design

Body and Seat:

To handle the high pressure and high temperatures, isolation valves must be designed to isolate high energy samples. As is established, elevated temperatures reduce strength of metals. Hence, metallurgy of the valve body is of prime importance. Globe valves with metallic seat are the most recommended. The valve should be designed to isolate small sample flow rates for sampling applications

Glands and bonnets:

In sample isolation valves for steam applications, valve parts are exposed to high temperature differences. To avoid sample leakages through glands it is recommended to use metallic braided graphoil packing rings. These glands must be packed in welded bonnets with top pressure plate on glands. The pressure plate can be tightened using bolts to ensure leak tight glands once valves exposed to steam.

Spindle:

Long spindles ensure good isolation from high temperature surface of valves and also less chances of heat conduction to handles. In many cases, particles are trapped in glands. Non -rotating spindles designed with thrust bearing, helps for better performance.


Forbes Marshall made Sample Isolation Valve is specially designed for this application

sampleisolationvalve

Robust Forge Body for high pressure and temperature steam application

Sample Isolation and Blow-down valves are designed in line with ASME PTC 19.11-2008 and ASME B16.34 standards

Non-rotating spindle design ensures easy operation

Braided Grafoil gland packing rings of high pressure and temperature applications in SWAS


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Author: Aditya Kanetkar, Product Manager, Forbes Marshall