**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 correctio**n 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 Nm^{3}/H =Kh_{C}^{0.5}

Q_{NR} is Flow meter reading in Nm^{3}/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/Cm^{2} abs.

P_{a} is the Actual Pressure Kg/Cm^{2} 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 Nm^{3}/H;

P_{D}=7 Kg/Cm^{2} abs

h=2500 mm WC.

K=40000/2500^{0.5} = 800.

What is the **correct reading** when the receiver shows as follows

32000 Nm^{3}/H and P_{A}=7.5 Kg/Cm^{2} g?

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

## Instrument Calibrations

DPT 0-2500 mm WC and PT 0-10 Kg/Cm^{2} 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 Nm^{3}/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:

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

**Author:** S. Raghava Chari

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