guidelines on installation of flow instruments

8/12/2019 Guidelines on Installation of Flow Instruments

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Installation Guidelines for C&I Installation of Flow Instruments

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General Guidelines on Installation of

Flow Instruments

• Extreme care shall be taken when welding and assembling the flowelement on the pipe. Any misalignment or rough particle or edge

inside the welded area may cause inaccuracy and this will increase asthe flow increases.

• Flow element s should always be located in upstream from any valve.

Downstream side of valve shall no longer be a homogenous mixtureand this may cause erratic behaviour of reading periodically.

• Care shall be taken while welding the impulse pipe. Improperarrangement of piping of DP instruments can create error in the

reading and even it gives an indication of negative flow of steam eventhough the flow is to be positive. Inadequate exchange of steam and

condensate in the piping may cause negative flow. The presence of

burr or dirt in the pipe can impede the flow of condensate back to thepipe, and when this happens, the pipe becomes full of water and hasthe effect of creating negative head.

• Always ¾” to 1” pipe is recommended for free flow condensate. Gate

valve shall be used for the tapping and pipe should be insulated uptocondensing pot.

• No obstruction of any type shall ever be installed closer than the limits

prescribed by manufactures. Even a small obstruction likeThermowell will disturb the flow sufficiently and the error will becomeappreciable.

• The Measuring instrument shall be located close to the flow sensing

element. The speed of response is reduced if there is a long run.

• The orifice plates shall be installed such that the extreme face is

perpendicular to the axis of the pipe within the +2 deg or – 2 deg.

and it should be ensured that when the extreme face is facing thedirection of flow, invariably the sign of positive (+) is marked on the

upstream.

• Location of Flow element should have clear straight run of 10D inupstream and 5 D in downstream.

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• For non-viscous liquid flow measurements, the best location for theinstruments shall be below the pipeline, if the instrument is above the

line, more maintenance will be involved. Suitable vapour taps shall beprovided.

• In the case of air and gas flow measurement system, as part of basicrequirement, it should be transmitted to the instruments without any

change in the differential head due to leakage and facility shall beprovided for checking the ‘O’ calibration.

• If the flow of any dry gases are to be measured, the location ofinstrument can be kept above or below the tapping points.

• For air flow measurements, it is always preferable to install the

instruments above the pipeline. Incase, if the instrument must beinstalled below the duct/pipeline, suitable Dust Collection Chambercan be installed as shown in the figure.

CONDENSATE POT

a) The basic concept of steam flow instruments impulse pipe are the

same as liquid flow instruments impulse pipe. However, additionalproblems are introduced in the steam flow measurement because of

the difference in density between the flowing fluid and the liquid in the

piping which makes condensate pot necessary and condensate pot willmaintain equal Heads of the liquid on the 2 sides of the secondary

measuring instruments.

b) The condenser Pot should be located nearer to the tapping point and

both condenser chamber should be at the level of upper tapping.

c) The unequal level will cause significant error due to false heads. Ifthe flow nozzle is installed in vertical pipe, the lower tapping pipe

which is bent and taken up to upper tapping in order to align with the

upper condensate pot, must be insulated, otherwise, error is createdwhen the bent pipe fills with condensate. The error may add orsubtract depending upon the direction of flow.

d) For flow measurements, the instruments should always be located

below the condensing pot, otherwise, the condensate will be lost fromthe system and the instrument will reach ‘O’ during the shutdown and

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the total system must be vented after the start up of the boiler inorder to remove Air and Vapour which might have got entrapped.

e) In an installation where the instruments must be located above the

tapping points and the condensing chamber should be equally locatedabove the instruments as shown in the figure, the pipeline upto thecondensing pot should be insulated.

SEALING CHAMBER

a) In the case of viscous fluids, flow measurements which are likely tofreeze or concealed in the pressure pipe or like such corrosive typefluids, suitable sealing chamber shall be used, the sealing liquid

should not mix or react with the medium to be measured.

b) The commonly used sealing liquid includes water, light oil, glycerol,ethylene glycol and mixtures of last two with water.

c) The sealing chambers, in each pressure pipe, should be installed at

the same level and as close as possible to the pressure tapings.

- The general arrangement for pressure tappings from the Sealing

Chamber to the instrument is shown in the sketch.

- The usual range of DP flow meter is 30 to 100 percent of rated

flow. Below 30 percent of rated flow, meter will not indicatecorrect reading. Therefore, if the DP is too small, to ensure good

performance, the instruments can be changed in order to bring thereading upscale. Rotameter can measure down to 10% of theirmaximum flow.

- The flow sensing device will give accurate results if properly

installed by adopting the standard precautions.

- Even after proper installation, periodical maintenance is necessary.

- The flow elements should be inspected before installation to find-out the presence of any corrosion / rusting or any blockage on thepressure tapping holes or any deposits on the face of the orifice

plate.

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