Installation

An in-line sensor or a tap to an adjacent mounted sensor must be located in a position where errors due to local disturbances, such as turbulence and vibration created by the process or adjacent machinery, are avoided For accuracy in lower pressure ranges, the sensor should include provisions for compensating for the weight of liquid in connecting lines so that the transmitted or observed pressure is that in the main piping or containment

An ANSI Piping Code recommends that the pressure take-off size not be less than %-in. pipe for operating up to
900 psi and 800°h and not less than %-in. pipe above these values. An acceptable method for installing a take-off is to weld a Weld-O-let or similar adapter to the main pipe or vessel and then drill through the adapter and pipe or vessel wall а ‘Д-іп.-diameter hole (or larger if desired) to produce a sharp clean edge at the inner wall. Actual si/e of the hole should be large enough to avoid plugging. Alignment of the axis of the opening should be perpendicular to the direction of flow to avoid false pressure readings due to impact velocity effects. Material specification and controls should comply with ASME Nuclear Piping Systems of proper class 1, 2, or 3.

The pressure sensor is mounted adjacent to the take-off in such a way as to reduce transmission of piping — or vessel-expansion strains, process heat, or system vibrations to the sensor mechanism Figure 4.22 illustrates a common

Table 4.17—Force Balance with Four Interchangeable Capsules*

Capsule

Range limit, t psi

Range — span limits, t psi

Max. over-range pressure, psi

A

-15 to 350

25 to 250

500

В

-15 to 750

50 to 500

1000

C

-15 to 1500

100 to 1000

2000

D

-15 to 3000

200 to 2000

4000

’With this design, for example, an A capsule may be adjusted for an operating range span of 210 to 250 psi with an expected accuracy to ±2 psi (+0.5% range span). A range span of —15 to +10 psig involves the lowest range and narrowest range span possible using an A capsule.

tBasic industry terminology is given in Bailey Meter Company Instruction Sec. 1І41-6.

image102
Подпись: Fig. 4.21—Sectional view of a force-balance pressure transmitter.

installation practice featuring y2-in.-O. D. tubing or equiva­lent pipe pitched to facilitate draining and maintenance. Full support of connecting tubing is recommended, un­supported tubing must take a lower pressure rating. Root valves at take-offs must be the same sue as the take-off. Above 900 psi and 800° F, the take-off may be swaged or reduced to allow a У2-іп. root valve, but the size at the main piping or vessel may not be reduced. Blow-down valves for drain must be at least У8-іп. pipe size. Instrument shut-off valves may be V4-in. pipe size and threaded to match standard instrument casing connections, this latter practice facilitates disassembly and routine maintenance and calibra­tion.

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