Feature Test & measurement • Reduced cost
• Faster and easier installation • Availability in a wide range of pressure classes and materials for compatibility with specific piping system designs • Fire safety ratings.
An alternative to the DBB is a monoflange, which is smaller and more compact, consisting of three needle valves – two block valves and a bleed valve. Needle valves have a smaller orifice than ball valves, so the monoflange is appropriate for lighter, less viscous fluids.
The classic DBB design is appropriate for all fluids, but especially for those with higher viscosity when using ball valves. In fact, a good DBB design should be ‘roddable’ (you can run a rod through to clear a clog) when system fluids are dirty or contain wax.
Impulse lines
Impulse lines connect the process interface valve to the manifold and transmitter and are used to convey the precise process conditions to the transmitter. When laying out impulse lines, three main objectives come into play: prevent corrosion or scaling; reduce leak points; and maintain tem- perature within a certain range. The first two are best achieved by using tubing and tube fittings made from an appropriate alloy. Stainless steel tubing can be bent and shaped, which reduces the number of mechani- cal connections. When mechanical connections are necessary, high-quality, two-ferrule, mechanical-grip type tube fittings will not back off with thermal cycling or vibration, unlike the threaded fittings used with carbon steel. The third objective is achieved by heating the impulse lines. You can insu- late your impulse lines manually – ‘field tracing’ – or purchase tubing that has been insulated in the factory and encased in a polymeric jacket. Pre- insulated bundled tubing comes ready to install in coiled lengths. It can be heated with steam or left unheated, as the application requires. It is important with pre-insulated tubing bundles to follow the manufacturer’s instructions for sealing the insulation when splicing into or cutting into the pre-insulated tubing bundle.
The manifold
The manifold consists of a set of valves whose bodies are machined from a single block of metal, usually stainless steel. The manifold mounts to the side of the transmitter and enables calibra- tion or service of the transmitter. Quality is especially important in a
manifold. During calibration or normal operation, at least one of the valves in the manifold is in the off position. If the shutoff is less than complete, the
30 Figure 3:
A double block and bleed valve can be constructed from three separate valves or can be purchased as a single,
self-contained unit
result is an inaccurate reading from the transmitter.
For example, when calibrating a transmitter in a standard differential pressure setup, the technician closes the two isolation valves and opens the equalization valve (Figure 4). This procedure results in the transmitter being ‘zeroed out’ for calibration. If one of the two isolation valves is leak- ing, however, the calibration will be unsuccessful and all subsequent readings will be inaccurate.
After calibration, the operator will
close the equalization valve and open the two isolation valves, placing the transmitter back in service. If the equalization valve does not achieve a positive shutoff, then high pressure will leak across the seat to the low- pressure side, rendering the differen- tial reading inaccurate.
These valves can leak due to debris, either scaling from corrosion or debris such as burrs. Burrs must be removed from all wetted surfaces as they can, for example, dislodge when the manifold is in service and catch in or scratch the valve stem or seat, preventing a positive shutoff. Another reason that valves leak is poor design. Most needle valves employ a metal-to-metal seal. The metal tip of the stem moves downward to seal against a metal seat of matching shape; and the tip of the stem may be shaped like a ball or a vee. In either case, it’s important that the tip does not rotate with the stem. If it does, it may grind into the seat, scoring it and cre- ating a pathway for leakage.
There are two main design approaches to achieving a non- rotating stem tip. In the case of a vee tip, there may be a knuckle joint that enables the upper stem to turn while the lower stem
remains stationary, except for axial (up and down) movement.
Figure 4: A manifold,
consisting of three needle valves, enables the technician to isolate and calibrate the transmitter in the field
In the case of a ball tip, a ball floats in the stem tip, like in a ballpoint pen. Among ball tips, there are designs that allow the ball to rotate on all planes, and others that restrict the ball, so it can rotate on only one plane. The latter is preferred. Each time a ball seals against a seat, a line seal is created where the seat meets the ball. If the ball is free to locate on all planes, it will develop mul- tiple, crisscrossing line seals, which will create leak points. If the ball can only rotate on one plane, it will develop only one line seal, with which it can seal repeatedly without leakage. In addition, a differential hardness between the stem tip and the seat is desirable in a needle valve because it aids in the process of forming a seal. The differential hardness allows the hardened stem tip to coin out imperfec- tions in the seat due to light contamina- tion or the buildup of a film on the seat. A low differential hardness between the stem tip and the seat can cause prema- ture failure due to the inability of the stem tip to coin out the seat, or from excess torque when closing the valve. The ball in a ball valve usually seals against a fluoropolymer seat. High pressure in the fluid stream pushes the ball against the seat, resulting in a seal. However, in a low pressure situation, additional force may be required for an effective seal. This force – sometimes referred to as ‘live loading’ – comes from a spring or O-ring inserted between the end screw and the seat. Not all ball valves contain these additional components, but they are valuable in achieving a positive shutoff in less than ideal conditions, when temperature and pressure are fluctuating. In valves that do not contain these components, an interference fit is usually relied upon to provide a low-pressure seal between the seat and the ball. Interference seal type seats typically do not offer low- pressure gas sealing abilities for more than a few actuations.
Raising the accuracy
So, if you value accurate measurements and specify a premium transmitter, then your instrument loop requires the same level of attention. Measurement accuracy is as dependent on the transmitter accuracy as it is on the quality of the instrument loop compo- nents, as well as how they are installed and maintained. Standardizing your facility on a core set of instrument loop details, quality compo- nents, and quality transmit- ters will raise the accuracy of your measurement.
Swagelok
www.swagelok.co.uk NOVEMBER 2013 Instrumentation Enter 667
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