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.
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
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
Author: Aditya Kanetkar, Product Manager, Forbes Marshall