# DP type level measurement & calculation

When a D.P. Transmitter is used for the purpose of measuring a level, it will be called a level transmitter. it is a common type of level measurement in industries. it’s level calculation is to simple that is given below.

## 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

Open tank level is a basic level measurement . its calculation is very simple that is given below . Before calculation there are some steps for transmitter mounting and also shown in figure-

1. Connect H.P. Tapping of transmitter to the base of tank.
2. L.P. tapping of transmitter open to atmosphere.
3. Now transmitter acts as simple pressure transmitter.

Pressure on H.P. tapping,  PH.P. = PAtm+(SG*H)

Pressure on L.P. tapping,  PL.P.= PAtm

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

ΔP = PAtm+(SG*H) – PAtm

ΔP   = SG*H

### Calculation For LRV

The tank will be empty to get LRV, pressure on HP tapping and LP tapping is measured. When the tank is empty, the pressure exerted on HP tapping and LP tapping will be 0. because liquid height will be 0.

Putting H = 0 in ΔP = PAtm+(SG*H) – PAtm

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

PL.P. = PAtm

LRV  =  PH.P. –  PL.P = PAtm – PAtm

LRV=0

### Calculation 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)

Range   =  0 to SG*H

## Closed Vessel Level Measurement

Close vessel level measured with two type:-

1. Dry leg level measurement
2. Wet leg level measurement

### Dry Leg Level Measurement

Level measurement in closed vessel done by dry leg method. This method is useful for all type liquids exclude vaporize. there are some steps to install transmitter and calculation formulas-

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

calculation :-

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

PL.P. = PAtm

ΔP = PH.P. –  PL.P. = (SG*H)

#### Calculation For LRV in Dry leg

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

PL.P. = PAtm

LRV  =  PH.P. –  PL.P = PAtm – PAtm

LRV=0

### Calculation For URV

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

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

PL.P. = PAtm

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

URV=(SG*H)

Range   =  0 to SG*H

### Wet leg level Measurement

Wet leg level measurement is used for vaporized liquid.

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.

calculation :-

PHP = Pgas + (SGP*H)

PLP = Pgas+(SGf*h)

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

ΔP(SGP*H)-(SGf*h)  or (ρp *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.

#### Calculation 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)

## Calculation 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 (ρp *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)}

### Calculation For LRV in suppression transmitter

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 {ρp *g*x}-{ρf*g*h+x)}

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

### Calculation For URV in suppression transmitter

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.

calculation :-

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

Plow = Pgas+(SGf*h)

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

orp *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.