Selecting
the Right Valve
for an Application
As
you get ready to specify or replace your next valve, first analyse your
system and consider these simple guidelines, designed to help you select
valves that will meet your unique system requirements,says Alok Gupta.
The
industry loses crores of ru-pees each year through the consequences
of improper valve selection. Improper valve selection can promote valve
failures, which can result in loss of system fluids, out of spec production,
downtime expenses, unsafe workplace conditions and environmental damage.
So,
how can one confidently select a valve that will install easily, perform
safely and reliably, and offer the lowest maintenance and overall cost
in the system? As you get ready to specify or replace your next valve,
first analyse your system and consider these simple guidelines, designed
to help you select valves that will meet your unique system requirements.
What
Type of Fluid will the System Carry?
Before
selecting a valve, consider the type of fluid the system will carry.
Is the fluid viscous or thin? Gas or liquid? Corrosive or inert? Such
variables can affect system components and operation. For example, fluid
viscosity affects system flow and valve requirements. Fluids that are
more viscous reduce system flow and leakage. On the other hand, a high-pressure,
light gas will move freely along its flow path, but can be more difficult
to seal.
Some
gases, such as hydrogen and methane, present significant ignition hazards,
and even the smallest leak to the atmosphere can be catastrophic. If
the system fluid is a toxic gas, such as arsine or phosphine, leakage
to the atmosphere can be harmful to plant personnel. Corrosive gases
or liquids such as hydrogen chloride, hydrogen sulfide, or even steam
can damage components and actually remove material by chemical or physical
attack.
What
are the System Operating Conditions?
System
operating conditions, such as temperature and pressure, are also important
factors in choosing a valve. For example, consider material selection
in high or low-temperature applications; component materials with varying
expansion rates can allow fluid leaks. Plastic components can shrink
and leak, or they can absorb water and other system media and become
brittle at low temperatures. Elastomers, too, can harden and crack in
cryogenic service, and they have high thermal coefficients of expansion.
In
addition, differential pressure can affect sealing capability. For example,
a system operating at 1000 psig can leak 10 times the amount of the
same system operating at 100 psig.
Will
the Valve be used in Severe Service?
If
you need a valve that will perform reliably in a severe service system,
consider a valve that is especially designed for that service, and confirm
that it meets current industry codes or standards. Below are a few examples
of applications and the corresponding recognised industry codes.
- Valves
used in fire safety applicationsFire Safety Specification API 607
- Valves
for sour gas serviceNACE (National Association of Corrosion Engineers)
Specification MR0175
-
Valves used in thermal fluid applicationsANSI/FCI 70-2 Specification
for leak-tight shutoff and a fire hazard standard like API 607
- Valves
used in chlorine systemsChlorine Institute Pamphlet #6, Piping Systems
for Dry Chlorine
What
Specific Valve Design Features will be required?
After
the fluid characteristics and operating conditions are examined, it
is also important to understand valve design features that are critical
to performance. While valve manufacturers cannot control the system’s
design parameters, such as the system fluid and operating conditions,
they can control design features that affect the valve’s performance.
 |
| Fig
1: (W-PH-0242) In conventionally packed valves, a PTFE packing cylinder
fits closely around the valve stem. When the packing nut is tightened,
the PTFE is forced outward against the valve bonnet and inward against
the stem to form a seal. |
One
important feature is the way a valve seals to atmosphere. Valves can
be packed or packless. Packed valves have either conventional or live-loaded
packing. In conventionally packed valves, a PTFE packing cylinder fits
closely around the valve stem (fig 1). When the packing nut is tightened,
the PTFE is forced outward against the valve bonnet and inward against
the stem to form a seal. Another design for packed valves is a live-loaded
seal (fig. 2). Live loading subjects the packing to consistent compression
that ensures it remains leak-tight, even in systems with frequent pressure
or temperature changes or high-cycle rates. Well-designed, live-loaded
packing exerts a minimum amount of pressure to achieve the sealwithout
increasing the amount of torque required for valve actuation. This way,
live loading also reduces wear and tear on the stem packing in high-cycle
applications. The two most common methods of live loading are an elastomeric
O-ring seal and spring-loaded plastic packing.
The
simplest live-loaded seal uses an elastomeric O-ring. The resilience
of the elastomer provides the live load. In the spring-loaded method,
a seal may employ plastic packing, but because plastics are not as resilient
as elastomers, a series of springs above a metal gland provide the live
load. A packing nut compresses the springs to maintain a more consistent
load on the packing.
...contd.
