Innovations in the design of primary isolation valves and manifolds for mounting pressure instrumentation can deliver enormous pressure control advantages to both instrument and piping engineers, ranging from significantly enhanced measurement accuracy, to simpler installation and reduced maintenance. Parker Hannifin has created a comprehensive range of instrument manifold mounting solutions for the main types of pressure instrumentation, employing close-coupling techniques which eliminate impulse lines and tube fittings to improve overall instrument performance and reliability.
There is no formal definition for close-coupling, but it has come to mean any instrument mounting system that enables a user to connect an instrument directly on to the process line, and primary flow control isolation valve. The overriding objective of this is to optimise the accuracy of measurement, by eliminating the long runs of tubing, tube fittings and bends and joints between process pipe and instrument that can cause pressure drops, and gauge/ impulse line errors.
Transmitter ‘hook-ups’ are often configured individually for each application, and can be large, heavy and difficult to install. By replacing such arrangements with purpose-designed close-coupled manifold/mounting solutions, users are able to optimise accuracy and reap a whole range of additional benefits such as:
increased transmitter accuracy and repeatability by eliminating impulse lines,
reduced size and weight, less stress on the process pipework,
easy winterisation, no heat traced impulse lines to power up or freeze,
faster installation, and the option of transmitter assembly and testing,
reduced maintenance, and higher instrument up time,
elimination of threaded connections,
simpler design with fewer components and fewer leak paths,
no risk of impulse lines blocking, as impulse lines are eliminated, and
pre-engineered compact design eliminates the need for impulse line field design.
‘Hook-ups’ for pressure transmitters often involve the custom configuration of complex arrangements of tubing, with multiple connections and valves. Measurement errors can be introduced as a result of long length impulse lines. These errors are frequently compounded by the use of different tube, fitting and valve components whose diameters may vary throughout an instrument installation.
Inaccuracies can distort the pressure impulse signal, causing errors of up to 15% (on flow measurements).
This traditional solution uses two sets of valve assemblies to create the double block and bleed valves, which are connected with impulse lines and connectors to the instrument manifold. It involves numerous discrete components, with all the associated costs and assembly time, and introduces bends that cause attenuation and turbulence that can affect measurement accuracy. If not carefully specified, other measurement accuracy problems can arise from differences in bore diameters of the various components, and unequal lengths of tubing.
Jim Breeze is product manager, Instrumentation Connections and Process Valves, Parker Hannifin, Instrumentation Products Division, Europe.