search

Каталог в формате PDF

Онлайн Обучение

Технический запрос

Онлайн-служба COT

PROCESS WORLDWIDE Jan 2014

Open the Valves of Performance
How to overcome the common problems with Instrumentation Needle Valves

PROCESS INDIA Volume 1 · Issue 5 · April 2014

Needle valves are versatile solutions for flow metering, as bleed valves or shut-off applications. The versatile components are a good choice for control and instrumentation solutions. The accurate needle valves market offers many types of needle valves – and the options yet increase. Finding and selecting the proper needle valve for a specific given application is therefor of crucial importance. If an improper valve is installed, the consequences can be quite drastic: Ranging from failure to comply with safety and industry standards via product contamination due to peeling of the valves coating materials up to a complete failure of the production process – any of the factors will ultimately lead to higher production costs due to frequent replacements of low-quality valves.

In fact, a single valve can make or break a whole process plant’s performance record. It is therefore crucial to understand the set-up and functionality of instrumentation needle valves to ensure a proper functionality. The problems with valve malfunctioning can be avoided only if the correct parameters re considered when selecting a component.

These include: ● Considering the specific application for your valve installation (pressure, temperature, media, flow, etc.). ● Considerations for using panels mounted needle valves. ● Sealing methods for leakage and wear prevention. ● Wear compensation (durability)

What Is an Instrumentation Needle Valve?

Needle valves are used for on-off service and the rough flow control of liquids or gases in pipes and tubing systems. These valves can be adjusted to enable higher or lower flow rate, using a needle which covers the valve opening by different degrees. This allows the user to control the size of the opening to regulate the fluid flow. Due to this design's versatility and clear construction, a user can choose from a variety of valves for a given application.

Although different set-ups and types of needle valves are available on the market,the basic design features are the same. In general, instrumentation needle valve characteristics are as follows: ● Size: Up to 1 inch (25mm), variation determines the available flow control. ● Pressure: up to 690 bar (10,000 psi) ● Temperature: up to 650°C (1,200°F) ● Configuration: straight or right-angle (90°) joint pattern ● Manual operation ● Safety requirements: proven design and burst pressure tests ● Proven raw material tractability ● Compliance with the following standards: MSS SP-99 (Instrument Valves) or equivalent; ASME B31.1 and ANSI/ISA 75.02 or ANSI/ISA 75.01 (Flow capacities)

Meeting the above standards is mandatory for any chosen needle-valve for any application. But with the diversity of metering solutions, it can be irritating to be spoilt for choice. Any valve selection has to be adapted to the specific required application and specific parameters of the process.

In any needle-valves catalog, operation pressure and temperature rating are presented for each valve. Familiarization of the specific process operating conditions - pressure and temperature is a must, in order to reach a successful decision about the appropriate device. The needle-valves catalog provides generally all the information required referring to the valve operation condition limits as well as its characteristics behavior along operation conditions changes.

Defining the Parameters for Choosing a Valve

It also provides the flow coefficient (Cv)Vs valve handle turns chart that presents the amount of flow control obtained for a given number of valve handle turns, meaning how many turns of the handle will result in the corresponding flow. Since the setting of needle valves can not be determined from the position of the handle, it is necessary to know how to read this chart in order to decide only on the needle valves that falls within the specific needs.

If a user needs need help with this information, top vendors are able to provide application or product engineering services intended to help navigate this information and make the most effective decision for product selection and implementation. When dealing with a quality vendor, it is recommended to consult with him prior to finalizing the needle-valve implementation. When a vendor is unable to provide this level of expertise consulting, it should be a first warn ing regarding the quality of the valves that are purchased from this vendor.

What to Consider for Panel Mounted Needle Valves

In many applications, a panel-mounted valve is required. Panel installation of a traditional needle-valve can be problematic, as it requires opening the valve assembly for the removal of the panel-nut to enable the valve’s bonnet mounting. Although seemingly simple, several aspects can cause problems with panel mounted designs: ● Opening the top packing-nut opens the actual packing sealing of the valve assembly and can invalidate manufacturer warranty.(The manufacturer cannot guarantee for proper valve closure by the user). ● Opening the top packing-nut may result in safety risk when the nut is not returned to its proper position and tightening torque. ● Therefore, for panel applications, it's recommended to use one of the needle-valves higher quality configurations, which use a male nut that saves the need of valve opening for removal of the packing-nut. This configuration is shown on the previous page. It eliminates the safety and quality risk, and ensures that the needle-valve warranty is kept valid.

Sealing Methods for Leakage and Wear Prevention

Needle valves have a relatively high sealing material wear: During regular steady state operation as well as due to initiated changes of the handle position, the packing sealing material and valve undergo stress and abrasion. Ultimately, the related wear reduces the effective lifecycle of the valve, influencing the process overall costs: Each valve replacement contributes to the process’ bill, not only because of the valve price and replacement coasts but mainly due to the delays of the served industrial production. A relatively simple way to delay the replacement of a needle-valve for some time is to tighten the packing nut (about 1/8 turn each time) when a packing leakage is detected.

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.

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.

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

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. Through a vast amount of studying and tests, it was found that hardness differentiation can be achieved through special heat treatment developed for stainless steel: Not the complete valve, but only the needle undergoes the hardening treatment that makes it three times harder than the needle housing. When the needle is inserted under pressure, no scratching is caused because as needle material is harder than the base. This fully-compliant solution has been successfully proven for eliminating the issues associated with wear resulted from identical hardness(of needle and needle housing) or needle coating peeling.

Although this treating causes some discoloration to the material, it does not affect the needle functionality or hardness. This heat treatment thus provides an advanced solution which avoids a lot of the potential problems common with needle valves.

To learn more, visit us at: www.ham-let.com

 

decoration
decoration

HAM-LET New Products

 

О компании:

Ведущий производитель клапанов и фитингов для трубопроводных систем, КИПиА и высокочистых производств. Мы используем свой опыт и мастерство для создания широкого ассортимента высококачественной продукции по международным стандартам.

 

 Смотрите наш корпоративный видео-ролик

 

Подробнее ››