EPS Leading meter2di Double Isolation Orifice Fittings

What Is a Double Isolation Orifice Fitting?

A double isolation orifice fitting takes the dual chamber principle one step further. In a standard dual chamber fitting, the plate is withdrawn into the access chamber while the line is under pressure, and that chamber is separated from the line by a single isolation valve. In a double isolation design, two independent isolation barriers stand between the access chamber and line pressure, with a bleed point between them. This arrangement is known as double block and bleed (DBB).

The purpose of DBB is not simply to install two barriers but to prove, before anything is opened, that isolation really has been achieved. If pressure does not build up again once the space between the barriers has been bled, both barriers are known to be tight. In EPS Leading meter2di this verification is made visible with HP and LP pressure gauges, and locking the valve and internal chamber shafts adds a physical layer of control.

meter2di is consistent with the HSG253 guidance on safely isolating plant and equipment and, by means of the DBB method, falls into the Category II proved isolation class. This matches operators' isolation philosophies that call for proved isolation before a pressurized chamber is opened, and it allows plates to be changed safely while gas continues to flow, without waiting for the annual maintenance shutdown.

Proved Isolation by Double Block and Bleed (DBB)

In a double isolation fitting, a plate change follows the classic dual chamber cycle with additional steps for proving and locking the isolation. The general sequence is:

1.      Moving the plate into the access chamber: After the access chamber has been equalized with line pressure, the drive shafts move the plate from the metering position into the access chamber. Gas continues to flow through the fitting without interruption.

2.      Closing the first and second barriers: The two independent isolation barriers that separate the access chamber from the line are closed in sequence. Each barrier is designed to hold line pressure on its own.

3.      Bleeding the intermediate space: The space between the two barriers and the access chamber are vented and depressurized. This step creates a safety buffer that stops a leak past either barrier from reaching the access chamber.

4.      Proving the isolation: The HP and LP pressure gauges are observed to confirm that pressure does not build up again. The gauge readings show the maintenance team and the permit-to-work authority that isolation has been achieved.

5.      Locking: The valve and internal chamber shafts are locked in position. This physical, visible control prevents a valve from being opened by mistake during maintenance and fits in with the operator's lockout procedures.

6.      Plate inspection and return: The access cover is opened and the plate is inspected or changed. The cover is then closed, the locks are removed, the barriers are opened in a controlled sequence and the plate is lowered back into the metering position.

Why a Single Barrier Is Not Always Enough

A small leak across the sealing face of a single isolation valve may go unnoticed until the access chamber is opened. On high-pressure or hazardous gas lines, many operators' isolation philosophies do not accept this risk. In a DBB arrangement, even if one barrier leaks, the intermediate bleed keeps pressure away from the access chamber and the leak becomes visible on the gauges. Where proved isolation is mandatory, a double isolation design should therefore be considered instead of a standard dual chamber fitting.

Effect on Turndown and Operating Cost

Because flow is proportional to the square root of differential pressure in orifice metering, the turndown obtainable with one plate is limited. Being able to change the plate safely and without interrupting flow makes it easier to move to a plate with a suitable beta ratio as flow changes, increasing the usable turndown of the station. Since orifice plates do not need flow calibration, this approach can be regarded as an economical alternative to ultrasonic and cone meters. Plate inspections that no longer depend on the annual shutdown plan also reduce operating costs.

Cycle Testing and Design Verification

Lasting safe isolation depends on the seals remaining tight over repeated opening and closing cycles. For this purpose meter2di has been put through an extensive program of cycle tests. Each unit is verified by finite element analysis (FEA) at design stage and pressure tested after assembly against industrial safety requirements and European pressure guidelines.

Key Features

·         Double isolation with visible proof: Isolation provided by two independent barriers and an intermediate bleed is verified in the field with HP and LP pressure gauges.

·         Lockable valve and internal chamber shafts: Every valve shaft and internal chamber shaft can be locked, holding the seals in their isolating position and providing a physical, visible form of control.

·         HSG253 Category II proved isolation: Using the double block and bleed approach of the safe isolation guidance, the design falls into the proved isolation class.

·         Plate change with uninterrupted gas flow: Plates are inspected and changed safely while gas continues to flow; the station does not need to be stopped.

·         Increased turndown capability: Safe, quick plate changes allow a switch to a suitable plate for different flow periods, extending the turndown.

·         Seals proven by cycle testing: Extensive cycle testing confirms that seals and hardware remain tight through repeated operation.

·         Isolation body verified by FEA and pressure testing: The body carrying the isolation valves and internal chamber shafts is analyzed by FEA at design stage, and every unit is pressure tested after assembly.

·         High-pressure solution: A high-pressure solution is offered for ANSI 2500# meters, with a duplex manifold rated to 10,000 psi.

·         Maintenance independent of shutdowns: Maintenance can be carried out whenever it is needed, without being tied to the plant's annual shutdown plan.

·         Complete metering solution: The fitting can be defined as one package together with a meter tube machined to suit it, a straightening vane or profiler, transmitters, a flow computer and an enclosure.

Technical Specifications

The values below show the typical design scope of the meter2di double isolation orifice fitting. The final configuration should be confirmed at project stage according to size, pressure class, material and the operator's isolation requirements.

Installation position, body material in the high pressure classes and manifold details are confirmed per project. The compatibility of the isolation arrangement with the operator's permit-to-work and lockout procedures should be verified before commissioning.

Key Advantages

·         Protection of maintenance staff: Proving isolation before the access chamber is opened reduces the risk of staff being exposed to line pressure and gas.

·         Uninterrupted gas flow, lower operating cost: Because gas keeps flowing during plate inspection, there are no costs from station shutdowns or bypass operation.

·         Flexible maintenance scheduling: Plate checks can be planned whenever they are needed, without being tied to the annual maintenance shutdown.

·         Wider usable turndown: Safe plate changes make it practical to adapt the plate to changes in flow, increasing turndown.

·         Primary element without flow calibration: Orifice plates do not require flow calibration; in life-cycle cost terms this offers an economical alternative to ultrasonic and cone meters.

·         Fit with the operator's isolation philosophy: Lockable shafts and a visible proving arrangement are consistent with the isolation and lockout rules of many operators.

·         Verified sealing repeatability: Cycle testing gives confidence in seal reliability at stations where plates are changed frequently.

Application Areas

Double isolation orifice fittings are used at measuring points where plates must be changed while flow continues and where isolation must be proved.

Gas Transmission Lines Requiring Proved Isolation

Preferred at transmission and distribution stations within the scope of ASME B31.8 when the operator's isolation philosophy requires DBB-proved isolation before a pressurized chamber is opened.

High-Pressure Measuring Points

Can be used at ANSI 2500# measuring points with the high-pressure solution and the duplex manifold rated to 10,000 psi.

Plants Operated Independently of Annual Shutdowns

At continuously operating plants with infrequent maintenance shutdowns, plate inspection can be carried out safely regardless of the shutdown plan.

Metering Stations with a Wide Flow Range

At stations whose flow changes from period to period, being able to change the plate safely helps keep the measurement within a suitable differential pressure range.

Sour Service Applications

With material selection in line with NACE MR0175, the fitting can be applied on lines carrying sour gas, where isolation safety is especially critical.

Criteria for Selecting a Double Isolation Fitting

Selecting a double isolation fitting concerns process safety and maintenance organization as much as metering engineering. Before a technical assessment, the following points should be clarified:

Isolation and Safety Requirements

·         The operator's isolation philosophy and the requirement for proved (DBB) isolation

·         Lockout/tagout and permit-to-work procedures; recording of pressure gauge readings

·         Safe destination for the vented gas

·         Whether a standard dual chamber or single chamber fitting would suffice if proved isolation is not required

Measurement and Flow Profile

·         Minimum, normal and maximum flow; seasonal flow variation

·         Target turndown and planned plate bores

·         Plate inspection frequency and the station's shutdown plan

·         Life-cycle cost comparison with alternative metering technologies

Pressure Class, Size and Material

·         Design pressure and availability of the required ANSI class in the chosen size

·         Need for the high-pressure (2500#) solution and the manifold

·         Body material suited to the minimum design temperature; sour service requirement

·         Seal material suitability for the fluid and operating temperature

System Scope

·         Meter tube machined to suit the fitting, with straightening vane / profiler

·         Differential pressure and process transmitters

·         Flow computer, enclosure and the maintenance mode approach during plate changes

·         Layout that gives maintenance access to the gauges and locking points

The fitting is a mechanical device; the hazardous-area suitability of the transmitters and flow computers connected to it should be assessed per project according to the area classification of the station.

Safe Isolation, Design and Measurement Standards

Together with the safe isolation guidance, the meter2di double isolation orifice fitting is designed within the framework of the following design codes and measurement standards.

HSG253 consistency relates to the design of the fitting; the safety of isolation on site also depends on the correct application of the operator's isolation procedures.

Safe Metering Solutions from TLY Enerji

In double isolation fitting projects, TLY Enerji aims to bring the metering team and the process safety team to the same table. By assessing your isolation philosophy, permit-to-work and lockout procedures, flow profile and pressure class together, we clarify whether meter2di or a standard dual chamber design is the right choice.

We coordinate plate bore planning, the definition of meter tube and transmitter scope, the flow computer maintenance mode approach, documentation, system integration and first start-up with your project team. We also help prepare a plate change procedure for your site teams that includes the isolation proving and locking steps. Our aim is that every plate change follows the same safe sequence, is properly recorded and preserves the continuity of metering.

Frequently Asked Questions

What is a double isolation orifice fitting?

A double isolation orifice fitting is a dual chamber fitting that separates the access chamber, into which the plate is withdrawn, from line pressure by two independent barriers with a bleed point between them. The isolation is proved with pressure gauges before the chamber is opened. The orifice plate can therefore be inspected and changed without interrupting gas flow and without exposing maintenance staff to line pressure.

How does double block and bleed (DBB) isolation work?

In the DBB method, two independent closure elements sit between the pressure source and the section to be opened. After both have been closed, the space between them is bled. If pressure does not build up again after bleeding, both barriers are shown to be tight. Even if one barrier leaks, the intermediate bleed prevents pressure from reaching the section being opened.

What does HSG253 Category II proved isolation mean?

HSG253 is guidance on safely isolating plant and equipment, and it groups isolation methods by the level of security they provide. Proved isolation refers to methods in which the effectiveness of the barriers can be verified before maintenance begins. Using the double block and bleed method, meter2di is classified under this guidance as Category II proved isolation.

How is isolation verified in the field?

On meter2di, isolation is verified by a visible proving arrangement consisting of HP and LP pressure gauges. After the barriers have been closed and the bleed opened, the gauges are observed to confirm that pressure does not rise again. In addition, every valve and internal chamber shaft can be locked, so the isolation is maintained physically and visibly throughout the maintenance work.

How does it differ from a standard dual chamber fitting?

Both designs allow the plate to be changed while the line is under pressure. In the standard design, a single valve separates the plate chamber from the line, and its tightness is not separately proved before opening. A double isolation fitting adds two independent barriers, an intermediate bleed, gauge-based verification and lockable shafts — which operators that insist on proved isolation require.

Can plates be changed without waiting for the annual shutdown?

Yes. Because meter2di allows plates to be inspected and changed safely while gas continues to flow, maintenance does not have to be scheduled into the plant's annual shutdown period. Plate checks can be made whenever needed. During the short time the plate is in the access chamber, the flow computer should be put into maintenance mode and the metering record managed according to the station procedure.

Why can orifice metering be an economical option?

Orifice plates manufactured to the standards do not need calibration in a flow laboratory; they are verified by dimensional inspection of the plate. Compared with technologies that need periodic flow calibration, this can reduce life-cycle cost. Carrying out plate inspections without interrupting flow also removes shutdown costs. For these reasons a double isolation orifice solution can be regarded as an economical alternative to ultrasonic and cone meters.

How does a double isolation fitting increase turndown?

In orifice metering, flow varies with the square root of differential pressure, so the flow range one plate can cover is limited. Because meter2di allows plates to be changed safely and without interrupting flow, switching to a plate with a suitable beta ratio as flow changes seasonally becomes straightforward. The differential pressure then stays within the transmitter's efficient operating band and the station's usable turndown widens.