Pitot tube blowback equipment


Thread Starter

Verhappen, Ian

I am in the process of installing a new pitot tube on one of our stacks which contains particulates. As a result, I am concerned that the pitot tube may become plugged with these particulates so would like to include an
automatic "blow back" controller on the impulse lines. Can anyone tell me who manufactures and off the shelf box that will do this?

- measurement is 0-25" WC
- isolate the pressure Tx and freeze the analogue signal during blowback
- blowback the pitot tube itself on a regular basis


Syncrude Canada Ltd.
PO Bag 4009, MD 0032
Fort McMurray, AB T9H 3L1
P 780 790-4079, Cell -799-6017
F 780 790-5190
[email protected]

Steve Myres, PE

Could you install a diaphragm over the end of the tube, which would transmit the velocity and static pressures into the tube but prevent entrance of particulates? The function would be kind of similar to a "gauge guard" on a fluid pressure gauge.

The diameter of the diaphragm cavity should probably be several times larger than the diameter of the pitot tube.
Although "off the shelf" custom transmitters units are available to do what you want, I think when you look inside, you will find they are over-priced. All they have is a couple of 3-way solenoid valves controlled by a timer, which isolate the PT and connect the impulse line upto an instrument air line. The timer also causes the PT to hold it's current value.

I believe you could achieve the same functionality by fitting the 3-way solenoid valves and controlling them via your DCS (or other control system that inputs the dP signal). The DCS knows when it is purging the lines, so it can choose to "freeze" the dP value internally. You then need a simple timer circuit to periodically perform the sequence:-

a) "freeze" dP input current value
b) Operate 3-way valves to put air onto line and isolate PT
c) After time, return 3-way valves to normal position.
d) Make dP input live again.

Pierre Desrochers

I have done this in the past with very good results. We have used a micro PLC and a set of 4 ball valves with actuators. Two valves to isolate the DP Cell lines from the main signal lines and 2 valves to introduce some compress air.

In the SCADA we would get a signal from this micro-PLC telling us that the purge is enabled. Then it would freeze the signal condition until the purge has ended. This took about 10 seconds. We would purge the 2 lines with 80
pounds of compressed air. Works good!

You really need some feedback limits switches to assure the vale position so the compress air does not destroy your DP Cell when it blows back...

The sequence is:

Close signal line to isolate DP Cell
LS1 confirms this
Open 2 compress air lines
LS2 confirms this
Timer ended
Close 2 compress air lines
LS2a confirms
Open signal line to isolate DP Cell
LS1a confirms this

With tryal we determined the purge to be needed every 10 minutes.

Pierre Desrochers
Montreal - Canada

Satish Salankimatt

I am working in 10 mw power plant and here the formula used to calculate compensated is

Qc = Qa* sqrt(((p+1.013)/(p1+1.013))*((t1+273)/(t+273)))

Qc is compensated flow
Qa is actual steam flow from tx.
p1 is orifice designed pressure.
t1 i orifice designed temperature.
p & t are actual pressure and temperature.

Kindly tell me whether this formula is correct.

Or help me by giving the correct formula for calculating the compensated
steam flow.

Thanks, Satish