Open the Valves of Performance

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Standard valves use a traditional technology of one molded layer of sealing material, fitted exactly to the sealed area. This valves design requires relatively high compression set to expand the packing ring enough for sealing. As a result, a high operating torque and increased wear of the packing ring are common with these units, causing outboard leakage following a relatively short period of use.

Modern valve designs, on the other hand, rely on use a configuration known as spring loaded chevron packing, where two coned packing rings replace the single solid packing ring. These two coned rings are located on one another in a cross-angle configuration, compressed by disc springs that enable easy handle turn, stable seal compression load and low sealing wear. This configuration actually converts the valve operation period to an extended lifecycle and higher reliability.

Damage to the Valve May be Result of Inadequate Material

When considering a proper needle-valve, a metal to metal shutoff sealing (metal stem-tip to metal orifice) is preferred. This set-up can be used in a wide range of relevant applications, as they combine high temperature tolerance, excellent chemical resistance and low problems with outgassing.

However, a relatively sharp orifice can potentially damage the stem-tip, while scratching it and may lead sometimes to galling phenomenon — no need to mention that this results by significant reduction of the valve lifecycle. These scratches are nearly inevitable when the needle and its base are made of the same materials and brought to contact under pressure.

Additional Information

Check List

Requirements for Instrumentation Needle Valves

Pressure and temperature operating range

Chemicals compatibility

Flow coefficient vs. handle turns

Panel mounting mechanism (for panel-mounted valves)

Sealing method and durability

Hardness differentiation for metal-to-metal shutoff mechanism

In the long term, a successful decision leads to lower costs.

A Question of Hardness — How to Make Your Valve Durable

Another potential difficulty may arise when both the needle and the housing are made of stainless steel. Such a case is a potential for cold welding (galling) under very high pressure operating conditions. In these cases, the needle and its housing may become stuck together due to seizure of the needle, rendering the entire valve useless. The obvious solution for creating a more durable needle is creating hardness differentiation. Material change however is going to change the overall valve profile which is unacceptable for most applications.

Different designs of valve stems provide optimum functionality for different applications.
(Picture: HAM–LET)

Some vendors use the coating solution, coating the needles with chrome. The disadvantage of this set-up is that the coating layer might be unstable and may peel. These coating peels flow under high velocity conditions in the system, potentially causing severe damage to other instruments in the industrial process (transmitters, flow-meters etc.). In fact, chrome coating might have issues with the industry RoHS standards.

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