Figure 1. Fluid Flow in a Pipe of Variable Size
In today's competitive market, every EPC company aims at building client's trust through each project it executes. One of the best ways to develop this trust is by putting yourself in client's shoe while working on revamp/ brown field projects, where the main intent of client is to safely use the existing facility, instruments, and installation as much as possible in order to reduce capital expenditure. This gives rise to the subject of adequacy check of inline/offline instrument, i.e evaluating if the existing inline/offline instrument shall work under new process conditions or not. This paper describes various steps involved for checking adequacy of existing orifice plates for Revamp / Brownfield projects. Since every project has a significant number of orifice plates, adequacy check of existing orifice is quite essential and can save a huge amount to client in terms of effort hours and installation cost, if existing plate is reused.
The term 'adequacy check' stands for verifying if existing instrument shall work efficiently under new process conditions or it requires replacement. Adequacy check for orifice plate examines the existing orifice plate for new flow range, rangeability, pressure, temperature, and pressure drop.
On revamp / brown field project, the main intent of client is to safely use the existing facility, equipment, instruments, and installation as much as possible in order to reduce capital expenditure on material, installation, commissioning, and spares. Thus keeping this in mind adequacy check is done for all inline/ offline instruments.
Although buying a new orifice plate does not cost much to the client, however, replacing an orifice plate not only includes the cost of plate, but also accounts for additional effort hours and installation cost. Since we have a significant number of orifice plates in any project, this may lead to a large amount, which can have commercial implications. Therefore adequacy check shall be performed.
Orifice plate is the most common type of primary element that is used for measuring the flow rate. It is used extensively as it has simple shape, high accuracy, and is easy to manufacture, although large pressure loss is inevitable (A Guide to Sizing Orifice Plate Flow Meters, n.d.). An orifice plate is simply a circular piece of metal with a specific sized hole bored in it. Most orifice plates are of the concentric type, but eccentric and conical (quadrant) and segmental design are also available.
(1) Refer Figure 1 below, the flow of fluid at any point in a pipe of constant cross-sectional area can be determined using equation of continuity, i.e.
Q = Av
where Q = flow rate;
A = cross-section area of the pipe;
v = average velocity of the fluid.
Figure 1. Fluid Flow in a Pipe of Variable Size
If the fluid is flowing in a pipe with different cross-sectional areas, i.e. A1 and A2, the continuity of equation will apply as:
Q = A1v1 = A2v2 (Liptak, 2003)
(2) Bernoulli Equation is based on the law of conservation of energy, which states that the total energy (Kinetic energy, potential energy, and pressure head) of a fluid or gas at any one point in a flow stream is equal to the total energy at all other points in the flowstream (Figure 2).
Figure 2. The Bernoulli Principle (Persamaan Bernoulli, 2018)
In other terms, this will be (Liptak, 2003):
(3) Whenever there is a flow restriction in a pipe, the following occurs:
Figure 3. Flow Profile through a Restriction in Pipe
Figure 4. Pressure Curve through Orifice Plate
By measuring the difference in fluid pressure across tapping upstream and downstream of plate, the flow can be obtained using Bernoulli equation and using coefficients established from extensive research (Miller,1996). In other terms:
where
∆ P = P1 - P2 ;
P1 = Pressure at Upstream;
P2 = Pressure at Downstream.
(4) Figure 5 shows the ratio of orifice bore diameter (d) to the pipe inside diameter (D) called the beta ratio (β)
β = d/D (ISO 5167-2: 2003)
The orifice plate has a typical bore diameter that ranges from 30% to 75% of the pipe inside diameter.
Figure 5. Square Edge Concentric Orifice Plate
With an orifice plate installed in a flow stream, increase in fluid flow velocity through the reduced area of the orifice develops a differential pressure across the orifice. The differential pressure generated is inversely proportional to the beta ratio of the orifice plate. Smaller the beta larger is the DP generated.
To size an orifice plate, the following shall be known.
i. Type of Orifice – Concentric, Eccentric, Segmental, Quadrant, etc.
ii. Process fluid – Liquid or gas
iii. Process parameters that include maximum flow (Qmax), normal flow, differential range, viscosity, inlet pressure, Specific heat ratio (Cp/Cv), and flow temperature.
iv. Pipe ID, element material, pipe material are available through P&ID and Piping Material Specifications.
v. Every orifice is sized while considering the base conditions (Standard Temperature and Pressure).
vi. Orifice sizing outputs are:
Adequacy check is performed to evaluate if an existing orifice plate will work efficiently under new process conditions. Following information and procedure are required.
1) Input the process parameters that include maximum flow, normal flow, viscosity, inlet pressure, Specific heat ratio (Cp/Cv), flow temperature, pipe ID, element material, pipe material.
2) Input the bore diameter of the existing orifice plate available in the As-built datasheet or through As-built pictures / drawings.
3) Calculate the following after feeding the above parameters.
where
Qv = Volumetric flow rate in m3 /s
Qm = Mass flow rate, kg/s
Cd = Coefficient of discharge, dimensionless
A2 = Cross-sectional area of the orifice hole, m2
P1 = Fluid upstream pressure, Pa
P2 = Fluid downstream pressure, Pa
ρ = Fluid density, kg/m3
β = Diameter ratio of orifice dia to pipe dia, and
∆ω = Overall Pressure loss, Pa
∆P = Differential Pressure, Pa
4) To evaluate if the existing orifice shall be suitable for new process conditions check, for the following:
Empirically, material of the orifice plate shall be of minimum SS316 or SS316L. However if process parameters changes, Instrument Engineer shall check with Metallurgy group and project specifications whether existing plate material is compatible to the new process parameters or not. In case material of higher grade is recommended, existing plate, impulse tubing, and element material of associated differential pressure flow transmitter shall be replaced accordingly.
It is imperative to calculate the turndown (Ratio of max flow to the min flow across orifice) of an orifice plate with new process flow values. Turndown shall conform to project's acceptance criteria. On most of the projects, accepted turndown is 3.5:1. However if new turndown is larger than that governed by project specifications, steps listed below are to be followed:
Due to change in process conditions, sometimes it is required to change the type of orifice plate instead of changing the flowmeter. For e.g. if the fluid is highly viscous and Reynolds number is less than 10,000, the quadrant orifice plate will be more suitable than square edge concentric orifice plate.
Consulting the manufacturer of existing orifice plate aids in getting to know of information and critical parameters of the instrument useful for evaluating the plate with new process conditions.
After identifying the plate that is to be replaced, it shall be demolished and complete procedure shall be followed for demolishing the instrument and the accessories associated with it. The confusion happens when revamp project does not provide any specification or clear guidelines for demolishing an instrument, which is a common scenario. In these cases, EPC needs to take a lead and mark clear status of all demolishing and re-used instruments in all the issued deliverables like P&ID, instrument index, datasheet, wiring, and loop diagrams. Few examples are listed below:
(1) Only plate to be replaced: Project guidelines shall be followed for removal of existing plate and installing a new plate at the same place when orifice flanges, tubing, and DP flow transmitter can be reused.
P&ID should clearly depict the replacement of orifice plate using tie-in points and note shall be added for recalibration of transmitter, for example refer the Figure 6 below. Instrument index status for orifice plate shall be marked for replacement and for transmitter as modification is required. Datasheet for orifice plate and transmitter shall be re-issued. No change in wiring and loop drawings.
Figure 6. P&ID Depiction – Replace Plate and Recalibrate Transmitter
(2) Use existing plate and recalibrate transmitter: None of the item shall be demolished. Plate, tubing, and transmitter shall be reused.
Note shall be added in P&ID for recalibration of transmitter, for example refer Figure 7. Instrument index status for transmitter shall be marked for modification required. Datasheet for transmitter shall be re-issued. No change in wiring and loop drawings.
Figure 7. P&ID Depiction - Reuse Existing Plate and Recalibrate Transmitter
(3) Plate and transmitter to be replaced: Both the items need to be replaced when the below listed conditions occur. All existing items – plate, tubing / capillary and transmitter shall be demolished and replaced by new plate, tubing / capillary, and transmitter.
Wiring can be reused based on the location of transmitter and junction box. Existing wiring shall be compatible with the new transmitter for reusing the same.
P&ID should clearly depict the replacement of orifice plate and transmitter using tie-in points, for example refer the below shown Figure 8. Instrument index status for orifice and transmitter shall be marked for replacement. Datasheets, wiring, and loop drawing shall be re-issued.
Figure 8. P&ID Depiction – Replace Plate and Transmitter
(4) Only transmitter to be replaced: In this case, it is advisable to replace the tubing as well. The reason is there are various factors of wear and tear associated with tubing when it is taken out. The tubing length may change based on the new location of transmitter.
P&ID should clearly depict the replacement of transmitter only, for example refer Figure 9 below. Instrument index status for transmitter shall be marked as replacement. Datasheet for transmitter, wiring, and loop drawing shall be re-issued.
Figure 9. P&ID Depiction - Reuse Existing Plate and Replace Transmitter
Below are the few case studies on Adequacy check of Orifice plates.
Refer serial no. 1 of Table 1, for a 4” makeup gas line, new flow rate has been decreased by 50%. Existing orifice plate installed in this line has bore diameter of 2.149” and beta ratio of 0.56. Existing transmitter had a range of -1000-1000 in H2O and was calibrated for range of 0-200 in H2O.
Table 1. Flow Meter Evaluation - Existing Meter with New Process Conditions
When adequacy check of the existing orifice plate is done, the following were noted:
Sizing output: Using the adequacy check procedure as described in section 2 of this paper, Maximum differential for meter max of 316421 sft3/h is 52.7 in H2O, normal differential is 36.62 in H2O. PP loss came out to be 1.23 psi (allowable PP loss = 3 psi).
Since DP range of 52.7 in H O falls within existing transmitter 2 range and PP loss is less than allowable limit, existing orifice shall work in new process conditions. However, transmitter was needed to be recalibrated for the lower range.
Also as described in clause 2 of section 4, the optimum solution was to reuse plate, tubing, and transmitter and that leads to no change in wiring and loop drawings.
Refer serial no. 2 of Table 1, for a 4” orifice plate installed in hydrogen service increased by 18%. Existing orifice plate installed in this line has bore diameter of 1.87” and beta ratio of 0.46. Existing transmitter had a range of -1000-1000 in H2O and was calibrated for range 0-500 in H2O.
When adequacy check of the existing orifice plate is done, the following were noted:
Sizing output: Using the adequacy check procedure as described in section 2 of this paper, maximum differential range for meter max of 984167 sft3/h is 2677.71 in H2O, normal differential is 2213 in H2O. Permanent pressure loss came out to be 73.5 psi (Allowable limit is 3.6 psi).
As clearly depicted by the result of evaluation, the differential pressure range for orifice plate exceeded the existing transmitter range and the maximum pressure loss was higher than the allowable pressure loss. Hence replacement of orifice plate and DP transmitter was recommended.
Wiring was reused as the location of transmitter and Junction box remains unchanged. For depicting the replacement on P&ID, appropriate tie-in points were drawn as described in clause 3 of section 4 of this paper.
Refer serial no. 3 of Table 3, for a 6” saturated steam line, new flow rate has been decreased by 82%. Existing orifice plate installed in this line has bore diameter of 3.75” and beta ratio of 0.618. Existing transmitter had a range of 0- 400 in H2O and was calibrated for range of 0-200 in H2O.
When adequacy check of the existing orifice plate is done following were noted:
Sizing output: Using the adequacy check procedure as described in section 2 of this paper, Maximum differential for meter max of 5359 lb/h is 6.19 in H2O, normal differential is 5.117 in H2O. PP loss came out to be 0.14 psi (allowable PP loss = 3.6 psi).
Since new DP range of 6.19 in H2O is much lower than existing DP range of 200 in H2O, it was recommended to replace the plate. Although DP range was well within the transmitter range, but this might affect the accuracy of transmitter. Transmitter was also replaced for the lower range in order to get good accuracy.
Wiring was reused as the location of transmitter and Junction box remains unchanged. For depicting the replacement on P&ID, appropriate tie-in points were drawn as described in clause 3 of section 4 of this paper.
Refer serial no. 4 of Table 1, for a 4” orifice plate, the installed vent gas service increased by 8%. Existing orifice plate installed in this line has bore diameter of 2.19” and beta ratio of 0.54. Existing transmitter had a range of -1000- 1000 in H2O and was calibrated for range 0-200 in H2O.
When adequacy check of the existing orifice plate is done, the following were noted:
Sizing output: Using the adequacy check procedure as described in section 2 of this paper, maximum differential range for meter max of 209868 sft3/h is 189.35 in H2O, normal differential is 156.5 in H2O. Permanent pressure loss came out to be 4.71 psi (Allowable limit is 3 psi).
Per result of evaluation the PP loss is higher than allowable limit therefore new plate of larger bore diameter was recommended.
Transmitter and Wiring were reused. Tie in points and notes were added on P&ID as described in clause 1 of section 4 of this paper.
Refer serial no. 5 of Table 1, for a 2” tray feed line, new flow rate has been decreased by 8%. Existing orifice plate installed in this line has bore diameter of 0.8168” and beta ratio of 0.421. Existing transmitter had a range of -1000- 1000 in H2O and was calibrated for range of 0-150 in H2O.
When adequacy check of the existing orifice plate is done following were noted:
Sizing output: Using the adequacy check procedure as described in section 2 of this paper, Maximum differential for meter max of 45 gpm is 260 in H2O, normal differential is 102.2 in H2O. PP loss came out to be 7.473 psi (allowable PP 2 loss = 5 psi).
Since DP range of 260 in H2O is too high for the existing orifice plate, it was recommended to replace the orifice plate. Transmitter was required to recalibrate and DCS range was reset to match new flow rates. Tie in points and notes were added on P&ID as described in clause 1 of section 4 of this paper.
Adequacy check of orifice plate is an essential need of hour. Since revamp projects provide less clarity about the adequacy check and demolishing procedure to be followed for orifice plates, this paper guides the EPC engineers about the correct procedure to be followed by quoting specific examples from different revamp projects.
Adequacy check for orifice provides a cost effective solution for brown field projects by reducing the capital expenditure, material installation, commissioning and spare. This works in best interest of client and leverages cost competitiveness of EPCs.