Boiler steam pressure

Lots of sensors work in absolute in a closed system.

Just depends what is wanted as a reading.

Some times the absolute is need if the system can go into a vacuum. Some times it is easier to figure out the saturation temperatures.

That would be alot easier, but we hv superheated steam. If you have any info regarding why absolute pressure is recommended. Please do share.

Adding the extra 1bar to get absolute increase the density of the mass flow, which is what we don't want because we are getting billed for the steam

The pressure units all depend on the calculations to get mass flow. In order to get mass flow you need pressure and temperature compensation.

You would need to see the back ground calculations. I would guess the built in steam tables are in absolute.

Now I understand your problem, if it is a matter to calculate the mass of steam that circulates through a pipe there is the absolute pressure that must be taken into account, according to Boyle-Mariotte law.

And you need temperature compensation too.

Theboss said:

That would be alot easier, but we hv superheated steam. If you have any info regarding why absolute pressure is recommended. Please do share.

Adding the extra 1bar to get absolute increase the density of the mass flow, which is what we don't want because we are getting billed for the steam

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Atmospheric pressure varies throughout the world as weather conditions and height above sea level etc. changes.
The only true zero (remains constant) is absolute zero. So all calculations for gas laws, steam tables etc. use absolute pressure.

That makes sense... For instance, measure pump pressure. Which is normally read as gauge pressure. Is that acceptable, if yes, why?

Theboss said:

That makes sense... For instance, measure pump pressure. Which is normally read as gauge pressure. Is that acceptable, if yes, why?

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Gauge Pressure is the normal process pressure standard for production units. As stated before engineering calculations and formulae are based on absolute pressure.
There are some processes which require absolute pressure measurement for technical reasons. Absolute pressure transmitters are used for such applications.
Gauge pressure is simpler to measure than absolute pressure.
Given that the earliest and simplest (and so cheaper) ways to measure pressure where inherently gauge pressure instruments then that is probably why gauge pressure was adopted as standard measurement of pressure.
Note that the common pressure gauge actually measures a differential pressure because the outside of the bourdon tube is at atmospheric pressure and the inside is at process presure. So the pressure gauge indicates pressure above atmospheric pressure. The variation in atmospheric pressure is usually insignificant when compared with process pressure. So you would not see any change in pg reading with variation in atmospheric pressure.

Gas calculations will be done with absolute pressure.

Now instead of buying multiple absolute transducers, you can use gauge transducers at your measurements point, then supply an absolute barometric pressure trasducer to read the site barometric pressure. You can then calculate the absolute pressure of each point by adding the barometer to it.
IE. Venturi_Inlet_Abs_P=Venturi_Inelt_Gauge_P + Barometer_Abs_P

Theboss said:

And if the boiler is a closed system, using the steam to create vacuum and steam tracing. Should we be measuring gauge pressure or absolute pressure of the steam?

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You do not say what you are trying to do on your steam system? "Creating a vacuum and steam tracing" is not very informative.

Generally many boilers are part of closed systems which supply steam to users for steam tracing, process heating, venturi's for pulling vacuum on processes etc. Steam condensate is returned to the boiler feedwater system where practical but there are always losses which must be made up by introducing additional feedwater to the boilers. Are you wanting to measure steam flow, enthalpy calculations or ?? For simple pressure measurement pressure gauges or gauge pressure transmitters are sufficient.

We trying to figure out why absolute pressure is measured on the steam line instead of gauge pressure. From what I have read absolute is a definite way of measuring pressure if its used for any calculations and gauge pressure is more for process pressure info.
Am I correct in saying so?

We using steam for Steam tracing and vacuum.

Theboss said:

That would be alot easier, but we hv superheated steam. If you have any info regarding why absolute pressure is recommended. Please do share.

Adding the extra 1bar to get absolute increase the density of the mass flow, which is what we don't want because we are getting billed for the steam

Click to expand...

You say that you are being billed for the steam.
So the reason for absolute pressure measurement is so that the Enthalpy of the steam can be calculated or looked up in steam tables. As you say that the steam is super heated then you must be measuring the temperature as well.
Both temperature and pressure are required to calculate / lookup superheated steam enthalpy. The steam flow must also be measured. Then you would be being billed on total heat value of steam being used.
Total heat value (KJ/Hr) = Flow (Kg/hr) * Enthalpy (KJ/Kg).

We are being billed according to the mass flow

Theboss said:

We are being billed according to the mass flow

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Ok then its simple. The pressure must be being used to correct the mass flow meter reading. The mass flow meter will have been designed for operating at a certain pressure. Actual deviations from the design pressure are being used to correct the mass flow back to design pressure conditions.

No. We using the pressure as an input back into the flow meter as a pressure reference to determine the density hence mass flow output.