UNDERSTANDING THE ENGINNERING DESIGN OF A PROPER ISOLATION SYSTEM

What is Isolation?

The secured separation of a complete Plant, equipment or service, a portion of piping from all forms of sources of energy is called ISOLATION.

Types of Isolation

Methods of isolation are generally divided into two categories:

a.Positive Isolation: This is to be applied where leakage cannot be tolerated due to safety reasons or for having safe work area within the plant.

b.General Isolation: For cases other than above, where isolation is required for a short period (e.g. control valve change out). General isolation may be provided by one of the following means:

- Double block and intermediate bleed (DBB) valve

- Single block and bleed downstream (SBB bleed) valve

- Single block valve (SB)

The bleed in the SBB arrangement allows for the safe disposal of fluids locked in between the block

valve and the point of positive isolation.

Isolation valves are to be located as close to equipment as practicable.

For systems isolation, isolation valves are to be located as close to headers as practicable.

POSITIVE ISOLATION

Positive isolation is an isolation achieved through the rotation of a spectacle blind to its closed position, insertion of a spade or the removal of a removable spool piece and the subsequent blinding of the open process piping. See Appendix 1 of this document.

It is provided where an isolated system or piece of equipment is required to be ‘opened up’ for maintenance, inspection or testing and modification works if any.

a.Equipment Isolation

Positive isolation of equipment is generally achieved by the closure of block valves plus the use of a spectacle blind or a spade and spacer arrangement. Removable spool pieces can be provided for vessel/tank nozzles > 3” including on the process feed / process liquid outlets / maintenance drains …process vapour outlet services, (i.e. not applied on nozzles dedicated to instrument / vent to atmosphere etc.). Removable spools are not provided with further positive isolation such as spectacle blind. Relief / blow-down connections are not provided with removable spools or spectacle blinds.

For all other static/rotating equipment such as separators, scrubbers, pumps and compressors, the requirement for spool pieces is to be determined when supplier information is available and detailed piping layout / routings are prepared. Removable spools and sometimes elbows may be required to

remove equipment for maintenance.

Otherwise spectacle blinds will be used except for large bore higher rated piping where spade and spacers will be used because of layout implications, b.System Isolation System isolation valves are also provided to enable positive isolation by spectacle blind rotation/ spade insertion or spool removal. System isolation valves may be located within equipment skids or packages, usually at the skid extremity or at the limit of a process train.

The advantages of providing positive isolation close to the system are as follows:

? Minimizes "blow down" volume on depressurization.

? Simpler for operator isolation monitoring and confirmation.

? The isolation should be visible from where the work is being performed.

? The valves may be placed within skids/packages, thereby conforming to the philosophy of maximizing skid-based equipment/bulks and minimizing the on-site construction requirement.

GENERAL ISOLATION

General isolation is defined as being isolation achieved through the closure of one or more isolation block valves. General isolation is provided to separate one process system from another and from any utility systems.

Shutdown valves can be considered to form a part of the isolation system, i.e. as one of the two block valves to satisfy a DBB requirement.

UNACCEPTABLE ISOLATION PROVISIONS

Double seated ball valves or double acting seals in a single valve body are not acceptable as double block for isolation purpose. Double block isolation must always be based on two independent valves.

Globe valves, control valves, relief valves and check valves may not be considered as part of an isolation system.

Where spare equipment is available (or) equipment can be by-passed (e.g. pumps, filters etc.), the relief valves do not require sparing and hence the relief valve is provided with an isolation valve when relieving into flare system. This is on the basis that the equipment itself can be isolated, drained and purged for relief valve removal.

In that case we need to ensure the integrity of the rest of the shutdown system must be ensured and not compromised. The valve in this case is to be disabled when being used for isolation purposes.

The use of shutdown valve as an isolation valve (one of the two block valves to satisfy DBB requirement) to be assessed on case to case basis and subject to approval.

The particular shutdown valve used as Isolation valve shall be completely disabled from the shutdown system in accordance with the applicable procedures and the integrity of the rest of the Shutdown System shall not be compromised.

SINGLE BLOCK / SINGLE BLOCK AND BLEED

A single valve isolation is to be provided for systems ANSI class 300 and lower non-toxic service. Where the block valve is not located directly at the blind/spool, consideration is to be made for the use of a bleed connection between the valve and the blind/spool on a case-by-case basis. The purpose of this bleed is to provide a means of depressurise or drain the volume between the isolation valve and the positive isolation element. In such bleed arrangements the bleed connection, if directed to safe location rather than a closed drain/flare, shall be equipped with two isolation valves for hydrocarbon or hazardous chemical service.

Single block isolation shall be limited to:

1. Single block valves to be used for fluids not classified as toxic, not containing hydrogen, fluids handled at or below Auto Ignition Temperature and fluids that are not flashing in pressure 300# and below.

2. Single block valves to be used for system containing fluid at temperature below their atmospheric Flash Point in pressure class 150#.

DOUBLE BLOCK AND BLEED

A double block and bleed isolation with two block valves with an inter-valve bleed to atmosphere (or drain/flare system as appropriate in toxic, hydrocarbon or hazardous service) is to be provided for ANSI class 600 systems and above for non-toxic, and all toxic service systems. The distance between the block valves is to be minimized to reduce the volume of fluid to be blow down through the bleed. For gas service the bleed will generally be located on the top of the pipework and for liquid service it will always be on the bottom. Where the double block and bleed valve set is not located directly at the blind/spool, consideration is to be made for the use of a bleed connection between the valve and the blind/spool on a case-by-case basis. The purpose of this bleed is to de-pressure or drain the volume between the isolation valve and the positive isolation element. In non-hazardous service a tundish is provided below a liquid bleed and connected to the appropriate open drain system if this is expected to be a routine operation. Where ever the Pressure Transmitters are located in accessible the instruments are mounted remotely in 2” pipe with 2 valve manifold. If the fluid service is toxic, then two mechanical isolation valve shall be provided.

DDB to be provided for the following

1. Double block valves to be used for system containing high temperature fluids at or above auto ignition temperature.

2. For piping, DBB isolation arrangement shall be separate valves with a bleed valve in between and for instrument, DBB isolation arrangement shall consist of integral block valve assembly of Two (2) isolation valves with bleed valves.

3. Bleed arrangements will require double bleed valves for toxic systems 300# and greater.

4. DBB to be used for Equipment and piping with high pressure 600# and above, irrespective of temperature rating.

5. DBB valves are to be provided at facility battery limit isolation between process plants. 6. DBB to be used for system containing fluids at or above their atmospheric flash point in pressure classes 300# and above.

BLINDED CONNECTIONS

In hydrocarbon or other hazardous service, connections for purging, flushing and local drains are provided with double block valves and a blind flange irrespective of piping class.

For non-hydrocarbon or non-hazardous service, connections for purging, flushing and local drains are provided with single block valves and a blind flange.

VENT CONNECTIONS

Except for tank roof vents, all other local vents on piping/equipment in hydrocarbon or other hazardous service are to be provided with two isolation valves and piping which routes the vent to a safe location irrespective of piping class. This includes vents from double block and bleed arrangements for pilot and burner gas. Tank roof vents and other local vents on piping/equipment in non-hydrocarbon or non-hazardous service are to be provided with a single isolation valve and blind flange only.

BLINDS/SPACERS

The use of a spectacle blinds or spacer/spade is dictated by the line size and flange rating.

Vent Isolation

a) All equipment, which are required to be isolated positively, shall be provided with a facility to depressurize to the flare header in a safe and controlled manner. Upstream and downstream valves shall be checked for leakage before the equipment is subjected to maintenance.

b) Pressure vessel or equipment in hydrocarbon, toxic service shall be provided with a manual vent connection around the relief valve bypass (where relevant) to the appropriate flare header to allow safe depressurization. It shall contain a manual globe valve and an upstream isolation valve to provide isolation from the flare system.

c) If manual depressurization (blow down) rate exceeds the flare system design capacity, the vent line shall be fitted with a restriction orifice to limit the flow rate.

d) Automatic depressurization (BDV) to flare system shall be provided as required by the relevant P&IDs, with a restriction orifice to limit the flow and back pressure.

e) Vents of air and water service (other than produced water and sour water) shall be vented to atmosphere. Vents shall be carefully located to avoid injury to personnel. Vents of nitrogen system shall be routed to elevated and well ventilated areas located away from operating platforms and walkways.

f) Vents / exhausts from equipment to atmosphere shall be located such that the exhaust gases are not recirculated to their air intakes of nearby buildings.

Tata Venkata Surya Prakash