Level measurement by using D.P. transmitter

When a D.P. Transmitter is used for the purpose of measuring a level, it will be called a level transmitter.

ABC of Level measurement by using D.P. transmitter

The pressure at the base of a vessel containing liquid is directly proportional to the height of the liquid in the vessel. This is termed hydrostatic pressure. As the level in the vessel rises, the pressure exerted by the liquid at the base of the vessel will increase linearly.

P = SG*H

where

P = Pressure (Pa)

SG = Std. gravity of process liquid  = ρg

H = Height of liquid column

ρ = Density

g = acceleration due to gravity (9.81 m/s2)

OPEN TANK LEVEL MEASUREMENT

    1. 1- Connect H.P. Tapping of transmitter to the base of tank.
    2. 2- L.P. tapping of transmitter open to atmosphere.
      1.          Now transmitter acts as simple pressure transmitter
      2.                          PH.P.     =        PAtm+(SG*H)

                                 PL.P.    =         PAtm

                    Differential pressure ΔP  =  PH.P. –  PL.P

      3.                                                          = PAtm+(SG*H) – PAtm

      4.                                                                   ΔP   = SG*H

    3.    

For LRV 

When Tank empty (H=0), calculate Transmitter Differential pressure (ΔP).

 

                          PH.P.     =        PAtm+(SG*0)                                                                                                              =      PAtm  

                          PL.P.    =         PAtm

                            LRV  =  PH.P. –  PL.P                                                                                         = PAtm – PAtm

                                          LRV=0

For URV

When Tank full filled with liquid (H), calculate Transmiiter Differential pressure (ΔP).

 

                          PH.P.     =        PAtm+(SG*H)                                                                                                     

                          PL.P.    =         PAtm

                            URV  =  PH.P. –  PL.P                                                                                         = PAtm+(SG*H) – PAtm

                                        URV=(SG*H)

calibration

Range   =  0 to SG*H

CLOSE VESSEL LEVEL MEASUREMENT

Close vessel level measured with two type:-

1- Dry leg level measurement

2- Wet leg level measurement

Dry Leg Level Meaurement

  1. 1- Connect H.P. Tapping of transmitter to the base of vessel.
  2. 2- connect L.P. Tapping of transmitter to the top of vessel
  3.  

calculation :-

Phigh = Pgas + (SG*H)

Plow = Pgas

ΔP = Phigh – Plow = (SG*H)

For LRV

When Tank empty (H=0), calculate Transmitter Differential pressure (ΔP).

                          PH.P.     =        Pgas+(SG*0)                                                                                                                =      Pgas 

                          PL.P.    =    Pgas

               

ΔP= LRV  =  PH.P. -  PL.P                                                                                       = Pgas -pgas                                          

                           LRV=0

For URV

When Tank full filled with liquid (H), calculate Transmiiter Differential pressure (ΔP).

                          PH.P.     =        Pgas+(SG*H)                                                                                                     

                          PL.P.    =         Pgas

                            URV  =  PH.P. -  PL.P                                                                                         = Pgas+(SG*H) Pgas

                                        URV=SG*H

Range  = 0 to SG*H

Wet leg level measurement

 1- Connect H.P. Tapping of transmitter to the base of vessel.

2- connect L.P. Tapping of transmitter to the top of vessel.

3- Fill LP impulse line with Known Density liquid according process demand.

  1.  

calculation :-

Phigh = Pgas + (SGP*H)

Plow = Pgas+(SGf*h)

ΔP = Phigh – Plow =  (SGP*H)-(SGf*h) or (ρp *g*H)-(ρf*g*h)

      ΔP(SGP*H)-(SGf*h)  or p *g*H)-(ρf*g*h)

 

Where:

SGP = Std. Gravity of process liquid.

SGf = Std. Gravity of filled liquid.

ρp= Density of process liquid.

ρf= density of filled liquid.

For LRV

When Tank empty (H=0), calculate Transmitter Differential pressure (ΔP).

calculation :-

Phigh = Pgas + (SGP*0)

Plow = Pgas+(SGf*h)

LRV= Phigh - Plow =  (SGP*0)-(SGf*h) or (ρp *g*0)-(ρf*g*h)

     LRV -(SGf*h)  or -(ρf*g*h)

For URV

When Tank full filled with liquid (H), calculate Transmiiter Differential pressure (ΔP).

calculation :-

Phigh = Pgas + (SGP*H)

Plow = Pgas+(SGf*h)

URV = Phigh – Plow =  (SGP*H)-(SGf*h) or (ρp *g*H)-(ρf*g*h)

      URV= (SGP*H)-(SGf*h)  or p *g*H)-(ρf*g*h)

Transmitter suppression

In some cases, it is not possible to mount the level transmitter right at the base level of the tank. Say for maintenance purposes, the level transmitter has to be mounted X unit below the base of a vessel.

Phigh = Pgas + {SGP*(H+x)}

Plow = Pgas+{SGf*h+x)}

  ΔP= Phigh – Plow =  {SGP*(H+x)}-(SGf*h) 

or {ρp *g*(H+x)}-{ρf*g*(h+x)}

  ΔP= {SGP*(H+x)}-{SGf*h+x)}or {ρp *g*(H+x)}-{ρf*g*(h+x)}

For LRV

When Tank empty (H=0), calculate Transmitter Differential pressure (ΔP).

Phigh = Pgas + {SGP*(0+x)}

= Pgas+(SGP*x)

Plow = Pgas+{SGf*(h+x)}

LRV = Phigh - Plow ={Pgas+(SGP*x)}-{SGf*(h+x)}or p *g*x}-{ρf*g*h+x)}

 LRV= {SGP*x)-{SGf*(h+x)}or (ρp *g*x)-{ρf*g*(h+x)}

For URV

When Tank empty (H), calculate Transmitter Differential pressure (ΔP).

Phigh = Pgas + {SGP*(H+x)}

Plow = Pgas+{SGf*h+x)}

 = Phigh - Plow =  {SGP*(H+x)}-(SGf*h) 

or {ρp *g*(H+x)}-{ρf*g*(h+x)}

 URV= {SGP*(H+x)}-{SGf*h+x)}or {ρp *g*(H+x)}-{ρf*g*(h+x)}

Transmitter Elevation (impulse line)

Transmitter mounted above the HP tapping point Is called Transmitter Elevation.

 1-Mount transmitter y above at H.P. Tapping of transmitter.

2- connect L.P. Tapping of transmitter to the top of vessel.

3- Fill LP impulse line with Known Density liquid according process demand.

  1.  

calculation :-

Phigh = Pgas + {SGP*(H-y)}

Plow = Pgas+(SGf*h)

ΔP = Phigh – Plow = {SGP*(H-y)}-(SGf*h) 

or                             {ρp *g*(H-y)}-(ρf*g*h)

      ΔP= {SGP*(H-y)}-(SGf*h)  or {ρp *g*(H-y)}-(ρf*g*h)

 

Where:

SGP = Std. Gravity of process liquid.

SGf = Std. Gravity of filled liquid.

ρp= Density of process liquid.

ρf= density of filled liquid.

Transmitter Elevation with Remote seal Transmitter

Phigh = Pgas + (SGP*H)

Plow = Pgas+(SGf*h)

ΔP = Phigh – Plow = (SGP*H)-(SGf*h) 

The differential pressure P sensed by the transmitter is always a negative number (i.e., low pressure side is at a higher pressure than high pressure side). 

ΔP = Phigh – Plow = -{(SGf*h)-(SGP*H)} 

      ΔP=-{(SGf*h)-(SGP*H)}