Pressure Correction Pneumatic Instrumentation

Instrumentation engineering root cause analysis (RCA) of flow transmitter pressure-temperature correction factors in pneumatic instrumentation.

Article Type:	Root Cause Analysis (RCA)
Category:	Instrumentation
Equipment Type:	Transmitters
Author:	S. Raghava Chari

Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.

Problem

The process licensor has forgotten to order and hence not available temperature transmitters threatened 6-months total plant commissioning delay – their rush order delivery time.

Author solution

The author recommended omitting the temperature correction attributing the below-given reasons:

Gas flow temperature correction is usually very small.

Example: correction factor at 50^o C gas temperature for orifice design 40^o C is just

=1-((273+40)/(273+50))^0.5 = 1.56 % only.

But calibrating pneumatic temperature transmitters for the required 0 to (273+60) K is very hard as sub-zero temperature baths are usually not available in most plants instrument workshops, hard to get.

Hence, flow temperature correction using pneumatic temperature transmitters introduces significant errors instead of refining the readings.

The plant where the author worked asked the author to remove both; however, they retained pressure correction only as the problems are absent with even pneumatic transmitters correction factors are significant.

The management accepted the recommendations and asked the author to configure the system for pressure correction only.

Also Read Theory: Flow Meter Pressure Temperature Correction

Gas Flow Meter Pressure Correction

The pressure correction equation is

Q_NC = Q_NR (hP_A /P_D)^0.5 ;    Equation 1; Symbols explanation follow.

Q_NC is Correct flow rate in Nm³/H =Kh_C^0.5

Q_NR is Flow meter reading in Nm³/H =Kh_R^0.5

Kh_C^0.5= Kh_R^0.5*(P_A /P_D)^0.5 Cancelling K we get

h_C^0.5= h_R^0.5*(P_A /P_D)^0.5

K is The flow installation constant 

h is the Differential pressure across the orifice

P_d is the Orifice design assumed pressure Kg/Cm² abs.

P_a is the Actual Pressure Kg/Cm² abs

Pressure Correction Pneumatic Instrumentation

The available analog computer’s one possible configuration is calculating h_C^0.5=(h*P_a)^0.5 offering the advantages of flow correction for pressure variations from orifice design pressure and 0‑100% easier to read flow receiver linear scale.

In addition, it eliminates the vendor given linearizing additional analog computers.

Q_NC=(h*P_a/P_D)^0.5. (1/P_D)^0.5 is a constant.

Hence include it as a second instrument scale factor.

The below given example illustrates it.

Example: Orifice Design Data as follows

Q= 40000 Nm³/H;

P_D=7 Kg/Cm² abs

h=2500 mm WC.

K=40000/2500^0.5 = 800.

What is the correct reading when the receiver shows as follows

32000 Nm³/H and P_A=7.5 Kg/Cm² g?

h^0.5=32000/800=40; h=1600 mm WC

Instrument Calibrations

DPT 0-2500 mm WC and PT 0-10 Kg/Cm² abs;

DPT output Signal Fraction (SF)= 1600/2500 = 0.64

PT output SF=(7.5+1.03)/10= 1.03 added to get abs pressure 0.853

Hence analog computer output SF = (0.64*0.853)^0.5  = 0.739

P_D SF=(7+1.03)/10 = 0.803

Hence 2^nd Scale Factor c = (1/0.803)^0.5 = 1.116

Hence Q_NC=1.116*0.739*40000;

as SF 1=40000 Nm³/H = 32988

% correction’n =100*(32988-32000)/32000 = 3.09%

Do Manual  =((7.5+1.03)/(7+1.03))^0.5 -1 = 3.06%

Both tally.

Author RCA Solution Benefits

Below given are the Author RCA benefits:

1. Min 6 months commissioning delays gone
2. Absence of temperature correction avoids future recurring problems and even plant shutdowns leading to its eventual removal
3. No wasteful TT purchase and installation expenses and subsequent removal after facing operation inconveniences
4. 4 spared analog computers, which can be configured for other instruments’ pressure correction tasks when needed.

Author: S. Raghava Chari

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