8.0 Guidelines For Take Off Connections For Sensing Lines

Following guidelines pertaining to the take off connections (pressure taps) should be followed.

8.1. LOCATION OF PRESSURE TAPS

a. In general the location of pressure taps in a pipe line having flowing fluid, should be at a point where the flow is uniform.

b. Location of pressure taps should be at least 5 diameters downstream from a symmetrical pipe fitting (as a reducer) and at least 10 diameters from any unsymmetrical fitting (as a tee, elbow or valve). It should also be at least 2 diameters upstream from any fitting or irregularity.

c. LOCATION OF PRESSURE TAPS IN HORIZONTAL PIPES/VESEL

I. GAS AS PROCESS FLUID: The location of pressure taps in the vertical meridian upwards is preferred mainly for wet gas. For following draining to occur, taps location angle should be less than 450 off the vertical meridian plane.

II. LIQUID AS PROCESS FLUID: The pressure taps should be located in a meridian plane with which the horizontal meridian is forming an angle not greater than 45O above or below according to the position of the measuring/sensing device.

If the liquid is clean, it is advisable to avoid the risk of gas in impulse line by using tap location below the pipe horizontal meridian plane. If on the other hand the liquid has significant solid content, then a position above the horizontal centre line is recommended. In neither case should the taps be more than 450 from the horizontal. In cases where there is considerable volume of gas in liquid line and needs special considerations a horizontal tap position should be used in conjunction with gas vent connection and gas collecting chambers in the impulse lines.

d. LOCATION OF PRESSURE TAPS IN VERTICAL PIPES/VESSELS

In case of vertical pipes/vessel, there are generally no problems as far as the radial position of pressure taps is concerned.

8.2. CONSIDERATIONS FOR PRESSURE TAP DESIGN

A. CONSIDERATIONS:

Following aspects should be kept in mind while designing the pressure taps for take off connections.

I. MATERIAL CONSIDERATIONS:

Take of connections at the source together with attaching bosses or adapters should be made of material at least equivalent to material of process pipes/vessels to which they are attached. They should be designed to withstand full line pressure, temperature and stresses.

II. DYNAMIC RESPONSE CONSIDERATIONS:

From measurement/accuracy considerations specially when the pipes/vessels contain compressible flowing fluids, the higher size of pressure taps are preferred for minimizing the possibility of plugging and for minimizing the error due to friction, inertia and lag in the connections while measuring dynamic pressures. When dynamics effects are not important smaller tapping size and impulse tubing may be used.

III. TURBULENCE ERRORS:

It may be noted that for measurement of static pressure in above case if higher size of opening is used as static pressure tap, the laminar flow past the tap is affected so that the velocity of tap opening is not effectively zero. Thus the pressure at tap involves some velocity pressure and is not true static pressure. Also large tap will produce eddies in the stream and as these eddies form and break away downstream, the pressure at tap fluctuates even when the undisturbed pressure is constant.

The turbulence error is also a function of edge condition of pressure tap hole. With the burrs up stream, flow is diverted away from the hole and the pressure sensed at the hole is less than the true static pressure. Conversely, if the burr is on downstream edge of the hole, the pressure at the hole will have a positive velocity effect and the pressure at the hole will be greater than the true static pressure.

IV. VELOCITY ERRORS:

In general the measurement errors are proportional to velocity pressure. Thus when stream velocity is high, it is necessary to take great care in tap construction with minimum possible tap diameter and special attention to the sharpness and squareness of the edges of the hole. When the stream velocity is low, the size and workmanship of the tap hole are not so important.

During fast transients, measurement errors may be more as the nozzle drop (take off connection losses) is proportional to the square of the velocity. However during steady state this loss can be neglected. Thus, in the system where fast transients are expected this requirement is more crucial. Thus based on above considerations the size of pressure tap holes should be decided depending upon the process fluid condition, reliability of measurement, accuracy, response time and requirement of physical strength.

8.3. RECOMMENDATIONS FOR PRESSURE TAP DESIGN

Briefly the design and sizing recommendations for pressure taps for different applications are given below.

a. In venturi tubes/orifice plates as the low pressure tap holes are to be located in high velocity region, it is recommended to use small size of pressure tap holes uniformly for HP & LP connections as recommended in ASME/PTC-9.5 (Flow measurement).

b. In all other applications where turbulence error does not come in picture, it is recommended to use 20 mm NB pressure tap holes from the consideration of reliable measurement/minimizing possibility of plugging/choking fast response and mechanical/physical strength.

c. Workmanship of pressure taps holes plays an important role in accurate/reliable measurement of pressure signals. Proper care should be taken while machining the pressure tap holes. There should be no burrs, wire edges or other irregularities on the inside of pipe/vessel at pressure tap connections or along the edge of the hole through the pipe/vessel wall.

d. In no case shall any fittings project beyond the inner surface of the pipe/vessel wall.

e. Also there should be no change in the pressure tap hole diameter for at least a distance of 2.5 d as measured from the inner surface of the pipe/vessels ( d =inside dia. of pressure tap hole).

f. Redundant taps should be located a distance apart such that the failure of one tap does not adversely affect the reading through the other taps.