marston technical catalogue

7/30/2019 Marston Technical Catalogue

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Bursting Discs - A bursting disc is a non-reclosing

device that is designed to burst or rupture at a

predetermined pressure, thus relieving a dangerous buildup of pressure or vacuum, protecting plant, pipework or

vessels from unacceptable levels of pressure or vacuum.

Conventional Discs

Often referred to as forward acting discs, ideal as a multi-

purpose and low cost solution.

Reverse Buckling Discs

These discs offer extended service life, particularly within

pressure cycling duties. Ideal for relief valve protection.

Graphite DiscsProvide ideal low pressure protection for highly corrosive

process media applications.

Explosion Vent Panels - Designed to provide low

pressure protection against the effects of dust or gas

explosions. In the event of an explosion a correctly sized

panel will open almost instantaneously to minimise the

effects of the blast.

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B u r s t i n g D i s c s &E x p l o s i o n Ve n t P a n e l s



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P R O D U C T R A N G E - F O R WA R D AC T I N G B U R S T I N G D I S C S

NT / NR Conventional Simple Domed

Assemblies.

The simplest of all discs, usually a single domed

metallic foil which will probably fragment upon disc

rupture.

NTG / NRG Conventional Grooved Disc

Assemblies.

A single metallic foil which has grooved lines of

weakness and is designed to be non-fragmenting.

It will normally withstand vacuum conditions without

the aid of a vacuum support.

CS Composite Slotted Disc Assemblies

A forward acting disc which has two membranes.A load-bearing slotted metallic outer membrane and a

weaker, usually fluoropolymer, seal membrane giving a

non-fragmenting design. This disc is widely used for

lower bursting pressures.

GR Graphite Discs

A flat graphite disc, impregnated with a high quality

resin, giving good corrosion resistance and low

bursting pressures. The unique GR arrangement is

designed to protect the disc from the effects of flange

bolt loading. This disc fragments on rupture.

Monobloc Graphite Discs

A flat graphite disc, impregnated with a high quality

resin, giving good corrosion resistance and low

bursting pressures. This disc does not require a

dedicated holder, and fragments on rupture.



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P R O D U C T R A N G E - R E V E R S E B U C K L I N G B U R S T I N G D I S C S

MN / MO Maxivent Assemblies

Usually a single foil disc which has the pressure applied

to its convex side. The dome inverts and is completely

expelled from its holder and stopped by an arrestor.

RBH Assemblies

Usually a single foil disc which has the pressure applied

to its convex side. The dome inverts and opens along

a peripheral groove. It is designed to be retained by its

hinge portion.

LRBH Assemblies

Usually a single foil disc which is designed specifically

for liquid duties. The pressure is applied to its convex

side. The dome inverts and opens along a peripheralgroove. It is designed to be retained by its hinge

portion.

SRBH Assemblies

This disc has two membranes. A load-bearing slotted

metallic membrane and a weaker, usually

fluoropolymer, seal membrane giving a non-

fragmenting design suitable for low bursting pressures.

RBX Assemblies

A single metallic foil which has pressure applied to the

convex side. The dome inverts and the disc opens

along radial grooved lines. It is designed to retain all

parts and withstand full vacuum without the aid of a

vacuum support.

GRB Graphite Disc AssembliesA unique disc manufactured from pure graphite

powder. It is suitable for high temperatures and

extremely low bursting pressures. This disc will

fragment on rupture.



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Safety valve service life can be prolonged

by using a bursting disc in series with the

valve. The disc can be designed to be

fitted at the inlet or at the outlet of the

safety valve.

Bursting discs are:

1) Virtually leak free.

2) Capable of preventing the process

media attacking the internal parts of

the safety valve, either by providing

corrosion resistance or a physical

barrier.

3) Suitable for protecting the vent side

of the safety valve from the ingress of

moisture and associated debris fromthe vent system.

Bursting discs are selected with a bore

size suitable for the appropriate inlet or

vent flange of the safety valve.

The free vent area of the bursting disc

used on the inlet side of the safety valve

is always substantially greater than that

of the valve.

A number of international standards are

available to allow the calculation of

suitable bursting disc sizes.

In all cases an excess flow valve is

advised to prevent any pressure build up

between the bursting disc and the safety

valve.

* Safety valves may also be referred to

as relief valves or safety relief

valves. Such valves are characterised by

their ability to relieve excessive

pressures at a pre-determined level and

to re-seal once that pressure has been

reduced to an acceptable, safe level. Such

devices provide re-closing pressure relief

and limit the quantity of product actually

released.

T H E P R O T E C T I O N O F S A F E T Y VA LV E S *



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Opening Type Vacuum Supports

Many simple conventional discs and most compositeslotted bursting discs are unable to withstand vacuumconditions without assistance.

To allow them to be used for duties where vacuum is

a possibility, even if only whilst equipment is beingcleaned, a vacuum support can be fitted. Usually this

takes the form of a multi-petal design Opening Type

Support, which when the disc bursts, opens up to

provide a large flow area.

The Vacuum Support is permanently fixed to the

bursting disc to ensure correct fitting. Therefore a

new support does have to be supplied and fitted with

each bursting disc.

When calculating the disc size required, the Free Area

through the support must always be considered.

Auxiliary Support

VA C U U M / R E V E R S E P R E S S U R E S U P P O R T S

In some applications, reverse pressures may existgreater than atmospheric pressure. Often an OpeningType Support alone will be sufficient. However, some-times an additional Auxiliary Support is required. Thisis designed to aid the opening support, whilst stillmaintaining a large free flow area. This type of supportis often used in double disc assemblies, where a sec-

ondary disc is used to prevent an often variablereverse pressure from affecting the performance of the primary disc. This is common when several burst-ing discs vent into a common line or flare stack. Thistype of support is generally re-useable, with only theopening support needing to be replaced.

Non-Opening Supports

Some discs, particularly graphite, use a permanent orNon-Opening Type Vacuum Support. These supportsare not usually attached to the disc but fit immediately

upstream of the bursting disc, sometimes in a recessprovided within the holder.

These supports follow the form of the disc, whether it

be flat or domed, and have holes through which the

product flows when the disc bursts. These supports are

considered to be re-useable and therefore only one is

required for each position.

More consideration must be given to the flow area for

these supports as a typical free area through a

permanent support is around 60%. See the relevant disc

brochure for more details.

Generally, reverse buckling discs do not require any

additional support to withstand vacuum or reverse

pressure.



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Holders

Insert type

Full face type

Pre-assembled holder

Pre-torqued holder

Protected dome

Unprotected dome

Lifting eyebolt

H O L D E R T Y P E S

The bursting disc holder can have a significant effect

on the performance of a bursting disc. The holder

provides an accurate location, sealing face, vent bore

size and form. Generally, holders will be

manufactured from stainless steel, though other

materials can be supplied when required. Holders

are normally non-torque sensitive.

Marston provide holders specifically designed for

each disc type, and to fit the particular application.

Usually the holder will be an insert type; one that fits

within the flange bolt circle. This provides an easier

method of fitting replacement bursting discs since

fewer flange bolts need to be removed. Full face

holders with flange bolt holes can be supplied if required.

Holders usually consist of two annular rings that

provide a flat sealing flange for the bursting disc.

Consequently, the performance of the disc should be

unaffected by excessive flange bolt loading. (Certain

graphite assemblies require close control of the

flange bolt torque.) Most holders are supplied with

assembly screws. These can be simply to hold theassembly together whilst fitting onto the plant, or,

for pre-torque type holders, they are tightened to

pre-set values to ensure an adequate joint is

achieved between the disc and holder before

installing onto the plant. More details of both types

of holders can be found in the individual product

brochures.

For sizes up to 250mm bore, the holder will usually

provide dome protection. Larger sizes are often

supplied without dome protection and where

conditions allow, simple clamp rings may be

acceptable. Where the bursting disc dome is not

protected, extreme care must be taken when fitting

to prevent damage.

For large assemblies, where the holder weightexceeds 25kg, or where it is considered beneficial

for handling purposes, Marston will make provision

for suitable lifting attachments.



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O P T I O N A L F E A T UR E S

Pressure Tappings

Tappings for monitoring equipment can be

incorporated in the vent-side of the holder, or in the

pressure-side if required. Any thread form and size is

usually possible but this may in some cases require an

increase in holder dimensions.

Corrosion Protection

For corrosive environments, holders manufactured

from resistant materials may prove costly. One

possible alternative offered by Marston is the Glass

Filled-PTFE insert that also provides a non-stick

surface. For extreme conditions a resistant metal

liner such as nickel or tantalum may be suitable.

Steam Heating

Where the duty may be prone to polymerisation the

holder can be heated. This can be achieved by

introducing a chamber around the holder body and

passing either hot water or steam through to prevent

the product from cooling and solidifying.

High Pressure

When the bursting pressures exceed the limits for

standard holder designs to retain the disc satisfacto-

rily, alternative designs are available. Wedge type

holders can be supplied on request but their perform-

ance can be affected by possible misalignment or

incorrect torque of the plant flange bolts. For this

reason, Marston would recommend the use of discs

with welded edge rings for high pressures that fit into

a simple recessed holder.



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The Marston ‘MAS’ eliminates the need to disconnect

the electrical supply to the rupture disc assembly,

reducing changeover time.

A ‘sensor’ is located in the vent side of the assembly,

retained by a screwed compression fitting. The hole

into which the sensor fits does not pass through to

the holder bore, therefore the sensor does not comeinto contact with the product contained in the vent

line. Attached to the vent side of the disc is a small

but powerful magnet. The magnetic field that it

generates is detected by the sensor. When the disc

ruptures, the magnetic field moves away and activates

the sensor. This simple switching effect can be used to

initiate an alarm or a programmed shutdown

procedure.

MagnetSensor

Retaining Screw

B UR S T D I S C I N D I C A T O R S

M A S ( M A G N E T I C A L A R M S Y S T E M )

M A S W I T H C O N N E C T I O N H E A D

Once a bursting disc has ruptured, it is often beneficial

to shut down relevant plant equipment as quickly as

possible. One common method of achieving this is to

fit a Burst Disc Indicator. A Burst Disc Indicator is a

simple circuit, usually fitted downstream of the

bursting disc. The signal is usually received in the

plant control room. This then instigates the shutdown

of the relevant equipment.

Recognising industries need to minimise maintenance

time, Marston now introduce the latest design in Burst

Disc Indication.

Where an exposed cable is unfavourable, the ‘MAS’

sensor is available with an integral connection head

where the wires terminate inside the head which is

rated IP68. The wires are never subjected to the often

harsh environment of an industrial plant; instead, they

pass through a connecting tube between the holder

and the connection head via an ‘O’-Ring seal.

The Marston ‘MAS’ provides an integral, leak tight

detector. The sensor, once fitted, does not need to be

renewed when replacing the rupture disc. It can be

unscrewed from the holder and replaced when the

new disc is fitted. When the disc is replaced, the new

disc includes a magnet already fitted which ensures

that it will be installed in the correct position.

Magnet Sensor

Connection Pipe Connection Head

‘O’-Ring



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B R E A K WI R E I N D I C A T O R S

R E M O T E I N D I C A TO R S

Marston also manufactures break wire type Burst

Indicators. Depending on their design they can either

be fitted directly to the bursting disc or fitted

‘remotely’ between the holder and the downstream

pipe flange. This type can be fitted to existing disc

assemblies or to safety relief valves.

Both operate in the same fundamental manner; a small

current passes around a normally closed circuit. When

the disc activates the flow breaks the membrane

carrying the circuit which causes the current to be

interrupted, indicating disc failure.

D I R E C T M O UN T E D I N D I C A T O R S

The direct mounted type has the circuit, or ‘Indicator

Loop’, fixed to the disc. The attached wires then pass

through the vent side holder, along a connection pipe

and connect to pins on a pressure tight, feed through

seal. Short wires on the other side of the seal then

terminate in the connection head.

Burst Disc Indicator fitted

directly to vent side of the disc.

The ‘remote’ burst disc indicator has the circuit fixed

to a membrane and is used on the downstream side of

the holder, replacing the usual gasket. The remote

design is not only available for new installations but

can also be fitted to existing equipment and can even

be used downstream of safety relief valves. Note that

remote burst indicators are not suitable for use with

Ring Type Joints or Tongue and Groove flange faces.

There are two types of remote burst indicator, the

traditional plastic membrane or a more robust metal

design. The metal design allows the use of corrosionresistant metals which permit their use at higher

temperatures.

Both of the ‘remote’ designs are available with a

connector mounted directly onto the unit with its

mating part fitted to a flying lead for permanent

installation into the plant. The wiring does not

therefore need to be disconnected from the plant*

when changing the burst indicator, simply unplug the

connector, fit a new burst indicator, and reconnect

the plug.

*Subject to plant and other local regulations and isolating

the feed if required.

Burst Disc Indicator

for remote fitting

between holder and

vent side flange.

All Marston Burst Disc Indicators have been approved

to II 1 GD 85°C EEx ia IIC T6 (-20°C ≤ Ta ≤ +75°C);

i.e. they do not induce or release sufficient electrical

energy when they function to cause an explosion even

in the most hazardous environment, Zone 0.

The system requires a 100mA maximum supply feed

from an appropriate isolator barrier.

Note:

When a Zener barrier is used, the holder must be earthedto inhibit high circulating currents.

Burst Disc Indicators are components which have been

considered NOT to require EMC testing on their own. It is

the users responsibility to ensure compliance with the EMC

Directive in relation to their particular system.

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