DP type level transmitter range calculations-part 3 closed tank

 

DPT range closed tank/calculation

This article explains range  calculation of  differential pressure transmitter for closed tank level measurements .If you want, read the basics and open tank level measurements here.

Open tank level measurements calculation

Basics of differential pressure type level measurements/Range calculation

 

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Data needed for calculation

To get the range  for the transmitter suitable for level measurement of a tank, ,we need certain data like 

1.density of the liquid,

2.minimum level position or position of 0% level

3.maximum level position or 100% position

4Height or position of minimum level (0%) from Transmitter HP port

5.filling liquid density ( if closed tank wet leg)

6.Liquid height  in wet leg

Note: The liquid density should be non variant for hydrostatic pressure type level measurement applications .

Calculation procedure

1.Calculate head pressure acting on the HP side when level is at 0%

2.Calculate the head pressure acting on the LP sidewhen level is at 0%

3.Find DP at 0%

             DP=HP -LP at 0% which is the LRV

4.Calculate the head pressure acting on the HP side when level is at 100%

5.Calculate the head pressure acting on the LP side when level is at 100%

6.Find DP at 100%

             DP=HP -LP at 100% which is the URV

For calculating pressure acting on  HP port and LP  port use equation

                   h×S.G

Note:In our calculations  HP and LP represents only  pressure acting at the transmitter’s  HP port and LP port respectively. And also HP side pressure may not be always greater than LP side pressure. see in wet leg calculations.

 So you just remember

 HP =pressure acting at HP port

LP= Pressure acting at LP port       

Example 1: Closed tank (dry -Leg)

 calculate range for the transmitter which is connected to measure level of a closed tank.The transmitter is mounted at the bottom and the Hp side tapping is taken from the minimum level position 0%.also transmitter is positioned at the minimum level.See below figure .

Dpt range closed tank

Find LRV - DP at 0%

To  find the lower range value calculate differential pressure when the level is at 0%.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 0%

Then LRV = DP at  0%

                   = HP-LP at  0%

 

 
 See figure above .Level is at 0% .we need to calculate the DP at this position which is the  LRV .

LRV=DP at  0%

Head Pressure on HP side

                         HP  =    h×S.G

                     
                                =   0 ×1.3
 
                                = 0 mmwc

Head Pressure on LP side

                       LP  =  0 mmwc

( Note:we need not consider vapour or gas pressure which develope  above  liquid surface as It’s effect will be cancel by connecting LP port to tank top)                                            

                                                   
                          DP    =  HP-LP
 

                                    =  0-0      

                                    = 0  mmwc

 

            At  0% ,    DP = 0 mmwc

            That is  LRV = 0mmwc

Find URV - Dp at100%

To find the upper range value calculate differential pressure when the level is at 100 %.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 100%

Then URV = DP at  100%

                   = HP-LP at  100%

Dpt range closed tank
 
 See figure above . Level is at 100% .We should calculate the differential pressure acting at this condition .

URV=DP at  100%

Head Pressure on HP  side

                     HP =    h×S.G

                            =  6000 ×1.3
 
                            =7800 mmwc
 

Head Pressure on LP  side

                  LP  =  0 mmwc

                                                   
                  DP    =  HP-LP
 

                            =  7800-0      

                            = 7800  mmwc

 

  At  100%  DP   = 7800 mmwc
 

    That is  URV  = 7800mmwc

 

 So we need to calibrate the transmitter with 

LRV=0 mmwc corresponds to output 4 ma

URV=7800 mmwc corresponds to output  20 ma

 

 

Example 2 Closed tank (wet leg)

We have to calculate range for the transmitter which is connected to measure level of a closed tank.The transmitter is mounted at the bottom and the Hp side tapping is taken from the minimum level position 0%.also transmitter is positioned at the minimum level.Lp side is filled with liquid .see  below figure for necessary  data

Dpt range closed tank

LRV DP at 0%

 

 find the lower range value calculate differential pressure when the level is at 0%.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 0%

Then LRV = DP at  0%

                   = HP-LP at  0%

Dpt range closed tank
 
 

LRV=DP at  0%

Head Pressure on HP side

                      HP  =    h×S.G

                     
                              =   0 ×1.2
 
                              = 0 mmwc
 

Head Pressure on LP side

                  LP  =    h×S.G

                   Note : There is always a constant head pressure acting on the LP side .This is due to the constant head of filling liquid in the LP side tubing .

                LP         =   7000 ×1.45
 
                         = 10150              

               DP    =  HP-LP
 

                         =  0-10150     

                         =  – 10150mmwc

 At  0% ,    DP =  – 10150 mmwc
 

  That is  LRV =  – 10150mmwc

URV= Dp at 100%

To find the upper range value calculate differential pressure when the level is at 100 %.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 100%

Then URV = DP at  100%

                   = HP-LP at  100%

Dpt range closed tank
 
 

URV=DP at  100%

Head Pressure on HP side

                             HP    =  h×S.G

                     
                                      =  6000 ×1.2
 
                                      = 7200 mmwc
 

Head Pressure on LP side

                            LP  =    h×S.G                

                                   =   7000 ×1.45
 
                                  = 10150mmwc

                        DP    =  HP-LP
 

                                 =  7200 – 10150  

                                = – 2950  mmwc

   At  100% ,    DP  =  – 2950 mmwc
 
 

       That is  URV  =  – 2950 mmwc

 So we need to calibrate the transmitter with 

LRV=  – 10150 mmwc corresponds to output 4 ma

URV=  – 2950 mmwc corresponds to output  20 ma

Example 3 Closed tank (wet leg)

We have to calculate range for the transmitter which is connected to measure level of  liquid in a closed tank.The transmitter is mounted at the bottom and the Hp side tapping is taken from the minimum level position 0%.also transmitter is positioned  1m below the  the minimum level.Transmitters Lp  side leg is filled with liquid . see below  figure for necessary  data.

 

 find the lower range value calculate differential pressure when the level is at 0%.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 0%

Then LRV = DP at  0%

                   = HP-LP at  0%

 
 

LRV=DP at  0%

Head Pressure on HP side

                      HP  =    h×S.G

                     
                              =   1000×1.2
 
                              = 1200 mmwc
 

Head Pressure on LP side

                  LP  =    h×S.G

                     
                         =   8000 ×1.45
 
                         = 11600            

 

               DP    =  HP-LP
 

                         = 1200-11600  

                         =  – 10400mmwc

 

 At  0% ,    DP =  – 10400 mmwc
 

  That is  LRV =  – 10400 mmwc

URV= Dp at 100%

To find the upper range value calculate differential pressure when the level is at 100 %.calculate the hydrostatic pressure acting on the high pressure side and low pressure side when level is at 100%

Then URV = DP at  100%

                   = HP-LP at  100%

 
 

URV=DP at  100%

Head Pressure on HP side

                             HP    =  h×S.G

                     
                                      =  7000 ×1.2
 
                                    = 8400 mmwc
 

Head Pressure on LP side

                            LP  =    h×S.G                

                                   =   8000 ×1.45
 
                                  = 11600 mmwc

 

                        DP    =  HP-LP
 

                                 =  8400 – 11600 

                                = – 3200 mmwc

 

   At  100% ,    DP  =  – 3200 mmwc
 
 

       That is  URV  =  – 3200 mmwc

 

 So we need to calibrate the transmitter with 

LRV=  – 10400 mmwc corresponds to output 4 ma

URV=  – 3200 mmwc corresponds to output  20 ma