Any bend in any pipe introduces more friction than would be the case were the pipe to be straight. In the case of an instrument, excessive bending could slow down the response of the transmitter to changes in the process variable. In the case of an instrument on a condensible fluid, like steam, the presence of excessive numbers of condensate loops begins to affect the accuracy of the instrument.

Hello "Guest",

Each bend has the potential to cause a shock wave in both directions along the impulse tube, during every change in pressure anywhere in the closed system.

Thus the more bends there are, has the potential to send multiples of shock waves in both directions, at time of an impulse.

Each individual shock wavefront has the potential to interfere with all others in the closed system.

Now you must see that the accuracy of any instrumentation, control devices etc becomes affected to a large degree, because of those shockwaves.

This is the reason for the explanation by PWSlack above.

Gentle curves: Yes.

Elbows and/or bends: No.

Kind Regards....

The number of bends depend on the length of the impulse tube. generally bends can be put at a distance of 1.5 mtr, depending on the application. Each bend carries a pressure drop, hence the minimum nos of bends, the better.

what is the procedure of pressure leak test for the impulse tubing...

1. All relevant activities i.e A. Test Pressure, B. Test duration, C. Test Medium, D, Pressure Drop shall be certified and recorded in the following formand shall be in line with ANSI B 31.3

2. Test on process line / Instrument air lines.

3. Ensure that the pressure applied is applicable for the instrument and should satisfy the operation forthe safety of instrument.

4. Check the manifold valve of low and High pressure is shut closed so that the applied pressure does noteffect the instrument.

5.The approval of the relevant "Installation Inspection ", obtained before starting the Pressure test of theimpulse lines.

6. Blowing by air of the impulse lines disconnected from block valves, to free from debris.

7. As test "medium" , for the pressure test of the instrument process leads (impulse lines), "Approvedclean water" shall be used.

8. Establishing of Test Conditions:

8.1 All impulse lines connections and fittings shall be exposed during the test

8.2 Test Pressure shall be set at (1.5 times Commodity Pressure)

8.3 Vacuum service Systems to be checked at 30 PSIG unless limited by a lower pressure design of the line.

9. Test results to be evaluated taking into account the pressure decrease that, in a consistent timeperiod ( say 3-5 minutes) should be neglible.

10. All tubing and instruments shall be well drained and dried after the completion of the test.

ITEMS TO CHECK (Impulse Tubing):

3.1 Material used according to specification

3.1 Steam tracing and insulation as per hook up

3.3 Seal fitting installed and Filled

3.4 Isolation and equalization valves / manifold installed

3.5 Vent and drain / manifold installed

3.6 Tubing slope Checked

3.7 Verify Tubing connection to HP & LP sides

3.8 Leak Test done

REGARDS

AHMED

INSTRUMENT QA/QC ENGINEER

its according to the location and the how much pressure is applied

Hi,

If the impulse tubing is routed from a dry gas line to the transmitter , and low points cannot be avoided, is it acceptable acc. to the standards? what are the other alternatives to it ??

The low points are unavoidable since there is a space constriant near the tapping point ??

regards,

Afifa