my qustion is, if i have a differential pressure transmitter installed on a tank, for convenience i need to shift the transmitter horizontally to a different location (i meant i am not shifting its point from the datum.), so i'm using a capillary. so does it make any effect on the reading? will the reading change if i use a capillary??

 

Theoretically, the fluid in the capillary should transmit the pressure the same as tubing.

But there are practical considerations.

You haven't mentioned what you have in the tank. The small ID of capillary is much more blockable by sludge than the larger ID of tubing.

The people who researched impulse tubing for high accuracy DP flow and produced a 'best practices' document, TUV NEL in the UK, recommend different sized impulse tubing for different media; small diameter for clean liquids, large diameter for dirty fluids
http://tinyurl.com/2colxrg

They also recommend not horizontal, but sloped impulse lines (downward towards the transmitter) with a minimum recommended slope of 8%.

Along with that recommendation is the advice to avoid high spots in sloped tubing that can trap gas. Tubing is rigid and will maintain its installed elevation (until it's abused) whereas the flexibility of capillary will allow it to get moved around and form gas trap high spot(s). Capillary used with remote seals forms a closed system, evacuated to vacuum and back-filled with fluid, so the issue of gas trap high spots is moot, there shouldn't gas in a closed system remote seal. But capillary open to the tank end could be subject to air intrusion even though its deadheaded.

The use of capillary tubing will act as a mechanical dampener to pressure change response. Gas bubbles in a liquid impulse line will cause additional sluggish response.

 

thanks david,

does the capillary effect and the surface tension make any effect on the reading??? the fluid that i wanted to measure is molten sulfur. to do that i'm using a chemical seal gauge. since its placed on a location a bit difficult for maintenance' i need to shift it to a bit to wards the walkway....

 

it may change slightly due to the difference in density of the fluid in the capillaries and that in the impulse lines.

 

they work fine in liquid sulfur applications, surface tension and viscosity have no visible effect.

 

If you have two remote seals, one at the bottom with another one at the top, then you can move the transmitter 'anywhere', below or above the datum and the LRV and URV do not change (assuming equal length capillaries).

If you have only the one remote seal at the bottom, then any deviation from the datum's elevation will be 'seen' as a shift up or down, requiring a corresponding LRV/URV suppression or elevation adjustment.

With a capillary remote seal, the capillary is not open to the tank, so you'll have seal/capillary temperature effects show up as level changes.

 

Any sulfur or sulfer vapor in the tube will freeze unless you keep the whole thing hot. And given Murphy's law that's almost certain to happen. I'd use something else. Especially since molten sulfur is fairly viscous even if you do keep it hot.

Regards
cww

 

if i move above or below, again i have to do calibration ryt. zero elevation/ suppression??

 

It depends on whether you have one remote seal on an open tank or two remote seals on a closed tank.