Selecting SF₆ Service Couplings: Plan Self-Sealing Function and Dead Volume for Minimal Gas Loss

→ Product category: SF₆ gas solutions

 

An SF₆ service unit is to be connected to a gas-insulated switchgear system. The system connection may, for example, have a larger nominal size, while the filling hose or measuring instrument is equipped with a smaller service coupling.

Mechanically, the problem can be solved with an adapter. However, for a technically sound SF₆ service connection, it is not enough simply to connect two threads.

Key factors include:

• connection geometry and nominal size,
• self-sealing valve function,
• dead volume of the connection,
• vacuum and pressure resistance,
• leak rate,
• seal and material compatibility,
• possibility of evacuation and gas recovery.

A suitable SF₆ service coupling must not only remain leak-tight during operation. It should also prevent unnecessary gas release and the ingress of air and moisture during connection and disconnection.

For such applications, ICS Schneider offers, among other products, SF₆ valves and couplings from WIKA. Particularly relevant are the GCx-08 connection parts with nominal size DN 8 as well as the corresponding GCx-06 and GCx-20 versions. Further solutions can be found under SF₆ Gas Solutions at ICS Schneider.

Why an SF₆ coupling is more than just a connection

For compressed air or water, a coupling is often selected mainly according to:

thread + nominal size + operating pressure

.

For SF₆, additional requirements apply.

The connection is part of a closed gas system

It must prevent:

• SF₆ from escaping into the environment,
• air from entering the gas compartment,
• moisture from being introduced,
• contaminants from entering the hose or system.

These requirements apply not only while the connection is coupled.

Especially during:

connecting → evacuating → opening → servicing → isolating → recovering → disconnecting

the design of the connection determines how emission-free the work can be carried out.

Selecting connection size and nominal diameter correctly

The nominal size largely determines the available flow cross-section of a service connection.

A larger nominal size can be advantageous for:

• large gas compartments,
• high gas flow rates,
• fast gas recovery,
• larger service units.

However, the largest possible coupling is not automatically the best solution.

For smaller:

• measuring lines,
• analysis instruments,
• filling sets,
• calibration setups

a smaller nominal size may be completely sufficient.

Additional components and unnecessarily large connection volumes can also increase the volume that must be evacuated or the amount of gas remaining after servicing.

The nominal size should therefore match the required gas flow and the existing connection system rather than simply being selected as large as possible.

Understanding DN 6, DN 8 and DN 20 correctly

WIKA SF₆ connection parts are available in different nominal sizes.

ICS Schneider offers, among others:

GCx-06 → DN 6

GCx-08 → DN 8

GCx-20 → DN 20

The nominal size is not automatically the thread size

The designation:

DN 8

describes the nominal size of the connection system and must not be equated with a thread designation such as:

G1/4

.

When selecting a connection, the following must therefore always be considered together:

• nominal size on the coupling side,
• connection thread or flange,
• mating part,
• adapter.

How self-sealing couplings work

Self-sealing valve and coupling systems are particularly important for SF₆ service applications.

The basic principle is:

disconnected → gas path closed

and:

fully connected → gas path open

During connection

the coupling halves are first connected gas-tight to one another.

Only during the intended coupling process is the gas path then opened.

During disconnection, the process is reversed

The self-sealing elements close the gas path before the connection is fully separated.

This significantly reduces open gas release compared with a simple threaded or hose connection.

Self-sealing does not mean zero volume

A small space may remain between the two shut-off elements.

This:

dead volume

must be taken into account when planning low-emission service work.

Why dead volume matters

Dead volume refers to a space within the service connection that is filled with gas but is not part of the actual usable volume of the system.

Dead volume can occur in

• coupling chambers,
• adapters,
• valve blocks,
• tees,
• measuring chambers,
• hoses.

When a connection is disconnected, gas in certain isolated sub-volumes cannot automatically flow back into the system compartment.

As a general rule

At the same pressure and temperature:

double the dead volume → approximately double the amount of gas

.

For frequently repeated service work, compact connections with the smallest technically necessary internal volume are therefore advantageous.

Self-sealing and low dead volume serve two different purposes: the self-sealing function prevents continuous gas release from the system and hose, while low dead volume reduces the amount of gas that can remain trapped between the closed valves during disconnection.

Why pressure affects the amount of trapped gas

Volume is not the only factor determining how much SF₆ is trapped in a service connection.

Absolute pressure is also relevant.

In simplified form, at constant temperature:

amount of gas ∝ absolute pressure × volume

This means

A coupling chamber at higher pressure contains more gas than the same chamber at lower pressure.

Therefore, before disconnecting it may be useful to:

• safely isolate the system gas compartment,
• recover gas from the service line,
• reduce the remaining line pressure in a controlled manner,
• only then disconnect the self-sealing coupling.

The exact procedure must always follow the instructions of the system manufacturer and the service equipment used.

Considering hose volume and coupling volume together

In practice, the small coupling chamber is often not the largest additional gas volume.

A long filling hose can contain significantly more volume.

After the system valve has been closed, SF₆ may therefore still remain in:

coupling + adapter + manifold + hose + service unit

.

This gas does not disappear simply because a self-sealing coupling is used.

The coupling only prevents the gas volume that remains under pressure after disconnection from being left open to the atmosphere.

For a low-loss setup, therefore

• hoses should not be unnecessarily long,
• unnecessary manifolds should be avoided,
• adapter chains should be kept as short as possible,
• residual gas from larger service volumes should be recoverable.

This topic is also covered in detail in the ICS technical article Evacuating SF₆ Filling Hoses Before Filling: Reliably Preventing the Ingress of Air and Moisture.

Evacuating the service line before opening the gas compartment

A gas-tight coupling prevents SF₆ from escaping, but it does not automatically prevent air from entering.

If an empty hose is simply connected

it initially contains:

• ambient air,
• moisture,
• possibly other contaminants.

If the SF₆ gas compartment is then opened, these substances can enter the system.

A technically sound service setup therefore allows the line to be evacuated

before the actual gas compartment is opened.

A typical setup can be, for example:

SF₆ gas compartment → self-sealing coupling → hose → manifold with vacuum connection → service unit

As a general procedure

1. the service line is connected gas-tight,
2. it is evacuated while the system gas path is closed or isolated,
3. leak tightness or vacuum retention is checked,
4. only then is the gas path to the SF₆ compartment opened.

This significantly reduces the ingress of air and moisture.

Disconnecting an SF₆ connection with minimal gas loss

The end of the service process must also be planned.

Depending on the specific system, a suitable general procedure is:

1. Complete the service process: Finish filling, measurement or gas recovery.
2. Isolate the system gas compartment: Close the intended system valve or shut-off element.
3. Consider residual gas: Recover SF₆ from the hose, manifold and larger intermediate volumes where intended.
4. Isolate the service line: Set the service unit or line valves to the intended position.
5. Disconnect the self-sealing coupling: Release the coupling according to the manufacturer’s instructions.
6. Fit protective caps: Protect the connection from dirt and mechanical damage.
7. Check the connection: After servicing, inspect the system connection for correct condition and, where required, leak tightness.

Simply closing the system valve and then disconnecting a long hose that is still at operating pressure is not a low-loss service concept.

Using adapters selectively

In existing switchgear systems, service equipment often encounters different:

• nominal sizes,
• manufacturer-specific connections,
• threads,
• flanges.

Adapters are therefore often unavoidable.

Long adapter chains become problematic

for example:

system → adapter 1 → adapter 2 → adapter 3 → coupling → hose

.

Each additional connection means:

• another potential leak point,
• additional sealing surfaces,
• additional internal volume,
• greater mechanical length and load.

Better is

an adapter that matches as directly as possible:

system connection → required SF₆ coupling

.

When selecting the adapter, not only the threads must match, but also:

• permissible pressure,
• vacuum suitability,
• material,
• seals,
• nominal size,
• leak rate.

Evaluating leak rate and helium tightness

A low leak rate is crucial for SF₆ connections.

The WIKA GCx-06, GCx-08 and GCx-20 connection parts are specifically designed for SF₆ gas applications.

For the GCx connection parts, ICS specifies, among other things

• vacuum and pressure resistance of -1 … +64 bar,
• operating temperature of -40 … +80 °C,
• 100 % tested helium tightness,
• for GCV, GCA and measuring chambers, a leak rate of ≤ 1 · 10-8 mbar · l/s.

Leak rate is not the same as dead volume

A coupling can be extremely leak-tight when closed and still release a small amount of gas from its internal coupling chamber during disconnection.

Therefore:

leak tightness + self-sealing + dead volume

must be evaluated together.

Considering sealing materials and decomposition products

The material of an SF₆ connection must be suitable for the specific application.

Factors to be considered include:

• pure SF₆,
• moisture,
• temperature,
• pressure,
• possible decomposition products after switching operations.

For WIKA GCx connection parts, materials used include

• aluminium,
• brass,
• stainless steel.

During maintenance work on equipment containing potentially contaminated or decomposed SF₆, the specified occupational safety and service procedures must also be followed.

Why protective caps are important

At first glance, a protective cap may appear to be simple accessory equipment.

However, on an SF₆ service connection it performs several important functions.

It protects the coupling side from

• dust,
• moisture,
• mechanical damage,
• contamination of the sealing surfaces.

Suitable versions can also provide an additional seal for the connection.

An unprotected SF₆ service connection should therefore not remain exposed to the plant environment for months or years.

Couplings for measuring and analysis instruments

SF₆ analysis often requires only comparatively small quantities of gas.

Particularly important here are

• small dead volume,
• short measuring gas lines,
• low leak rate,
• fast and reproducible coupling,
• suitable adapters for the system connection.

An oversized service line can create an unnecessarily large gas volume for a small measuring task, which then has to be evacuated before measurement and handled afterwards.

For analysis and smaller service connections, WIKA connection parts with:

DN 6

or:

DN 8

may therefore be suitable, provided they match the required gas flow and mating connection.

Couplings for filling and topping up

During filling or topping up, the service line connects an SF₆ gas supply to the gas compartment of the system.

A possible setup is:

gas cylinder → pressure regulator → SF₆ hose → self-sealing coupling → system

The service coupling must

• be suitable for the pressure involved,
• be vacuum-resistant if the line is evacuated beforehand,
• operate gas-tight,
• match the hose system,
• allow disconnection with minimal gas loss.

A complete system for this task is, for example, the WIKA GFK-10 Filling Set for SF₆ Gas.

The filling set uses self-sealing valves and is designed for filling and topping up SF₆ gas compartments.

Couplings for gas recovery

During maintenance or decommissioning, existing SF₆ should not simply be released from the gas compartment.

It is recovered using suitable service equipment and transferred to an intended storage container.

In this case, flow resistance becomes more important

For large gas compartments, a connection that is too small can increase recovery time.

The nominal size must therefore always be considered together with:

• gas compartment size,
• hose length,
• service unit,
• required gas flow.

For larger service tasks, connection parts with:

DN 20

are also available.

Typical selection and installation errors

Observation	Possible cause	Recommended check
Coupling does not fit mechanically	incorrect nominal size or connection geometry	check DN, thread and mating part completely
Several adapters required in series	service connection not planned appropriately	select a direct adapter
Gas can be heard escaping during disconnection	service line still under pressure or unsuitable coupling system	check isolation and recovery procedure
Some gas is released despite self-sealing	gas trapped in coupling dead volume	evaluate dead volume and disconnection procedure
Large amount of gas remains after servicing	long hose or large manifold still under pressure	recover residual gas and reduce volume
Moisture value deteriorates after topping up	service line not sufficiently evacuated before opening	perform evacuation and vacuum retention test
Coupling leaks after repeated use	sealing surface contaminated or seal damaged	clean connection or inspect seal
Service takes unusually long	nominal size or hose cross-section too small	evaluate the complete flow path
Analysis consumes unnecessarily large amounts of gas	oversized hoses and excessive dead volume	shorten measuring gas path and reduce volume
Service connection is contaminated	protective cap missing	clean and inspect connection and use protective cap
Connection is pressure-tight but not suitable for vacuum	unsuitable standard coupling used	check SF₆-specific vacuum and pressure approval

Practical example: DN 20 system with DN 8 service line

A gas-insulated switchgear system has an SF₆ service connection with:

DN 20

A compact service or measuring system with:

DN 8

is to be connected for maintenance work.

Unfavorable solution

Several available universal adapters are combined:

DN 20 → threaded adapter → intermediate piece → another adapter → DN 8

The connection may work mechanically, but it has:

• more sealing points,
• greater dead volume,
• greater installation length,
• more potential leak points.

Better solution

A direct adaptation intended for the SF₆ system from:

DN 20 → DN 8

is used.

A suitable SF₆ hose is connected to the DN 8 side.

Before opening the system gas compartment

the complete external volume consisting of:

adapter + coupling + hose + manifold

is evacuated.

After servicing, the gas compartment is isolated again and the SF₆ remaining in the hose is recovered as far as intended.

Only then is the self-sealing coupling disconnected.

Correct adapter selection therefore not only reduces installation effort, but also the number of sealing points and unnecessary service volume.

Systematically selecting an SF₆ service connection

1. Identify the system connection: Determine manufacturer, coupling system, nominal size and connection geometry.
2. Define the service task: Filling, topping up, analyzing, evacuating or recovering.
3. Determine the gas flow: Derive the required nominal size from the service task and gas compartment size.
4. Determine the mating part: Check the connection on the hose, measuring instrument or service unit.
5. Provide self-sealing function: Use suitable self-sealing valves and couplings for recurring SF₆ service connections.
6. Minimize dead volume: Avoid unnecessary adapters, large manifolds and excessively long hoses.
7. Check vacuum suitability: The service connection must also function reliably during evacuation.
8. Check operating pressure: Design all components for the maximum pressure that can occur.
9. Check leak rate: Use SF₆-specific gas-tight components.
10. Check materials: Consider SF₆ and possible decomposition products.
11. Plan evacuation capability: Ensure the service line can be evacuated before opening the gas compartment.
12. Plan gas recovery: Do not unnecessarily release residual gas from the hose and larger intermediate volumes.
13. Provide protective caps: Protect coupling surfaces during storage and operation.
14. Reduce adapters: Use the most direct possible connection between the system and service connection.
15. Document the service procedure: Clearly define coupling, evacuation, opening, isolation, recovery and disconnection.

Suitable SF₆ products from ICS Schneider

WIKA GCx-08 – DN 8 connection parts

For many service and measuring applications, the WIKA Type GCx-08 is particularly suitable.

The range includes:

• self-sealing valves,
• matching couplings,
• protective caps,
• fittings,
• adapters,
• connections for analysis instruments.

ICS specifies, among other things:

• connection and disconnection under pressure and vacuum,
• vacuum and pressure resistance of -1 … +64 bar,
• operating temperature of -40 … +80 °C,
• 100 % tested helium tightness.

WIKA GCx-06 and GCx-20

For other flow and connection requirements, corresponding connection systems with:

DN 6

and:

DN 20

are available.

An overview can be found under Valves and Couplings for SF₆ Gas Systems.

WIKA GCH-08 and GCH-20 SF₆ hoses

The WIKA GCH-08 and GCH-20 are hoses specifically designed for SF₆ service work.

The hoses:

• are vacuum- and pressure-resistant,
• are tested for helium tightness,
• have self-sealing valves and couplings,
• are available in DN 8 and DN 20.

WIKA GFK-10 Filling Set

For filling and topping up, the WIKA GFK-10 Filling Set is available.

It is designed for controlled filling of SF₆ gas compartments from a gas cylinder and uses self-sealing valves.

WIKA GVP-10 Vacuum Pump

If a service line or gas compartment must be evacuated before filling, the WIKA GVP-10 is one suitable option.

The suction connection is designed for a DN 8 hose.

Further products can be found under SF₆ Gas Solutions at ICS Schneider.

Conclusion

An SF₆ service coupling should not be selected solely according to thread or outside diameter.

The nominal size must match the service task

Large gas compartments and high gas recovery rates may require a larger flow cross-section, while measuring and analysis tasks can often be carried out with smaller nominal sizes.

Self-sealing reduces gas loss during connection and disconnection

With suitable coupling systems, the gas paths are automatically closed as soon as the connection is disconnected.

However, self-sealing does not eliminate dead volume

Gas trapped between the shut-off elements or in hoses, adapters and manifolds must be considered separately.

Small dead volumes reduce the amount of trapped gas

Especially for frequent service and analysis operations, it is therefore advantageous to keep the entire additional gas path compact.

Service lines should be evacuated before opening the gas compartment

This prevents unnecessary ingress of air and moisture into the SF₆ system.

Residual gas in the hose should not be regarded as an unavoidable loss

For larger service volumes, a gas recovery option should be provided.

Adapter chains should be avoided

A directly matching adapter reduces sealing points, installation length and additional internal volume.

For practical applications

Identify the system connection → determine the service task and required flow rate → select the appropriate nominal size → use a self-sealing SF₆ coupling system → check pressure and vacuum suitability → consider leak rate and materials → keep dead volume as small as possible → avoid adapter chains → evacuate the service line before opening the gas compartment → isolate the gas compartment after servicing → recover residual gas from larger service volumes → disconnect the self-sealing coupling → fit the protective cap → check the connection for correct condition.

FAQ: Selecting SF₆ Service Couplings Correctly

What is an SF₆ service coupling?

An SF₆ service coupling connects a gas compartment to a hose, measuring instrument or service equipment and is specifically designed for gas-tight handling of SF₆.

Why can I not use a standard compressed-air coupling?

SF₆ service connections must, among other things, be designed for high gas tightness, vacuum operation, pressure operation and suitable material compatibility. A standard compressed-air coupling does not automatically meet these requirements.

What does self-sealing mean?

With a self-sealing coupling, the gas paths are automatically closed during disconnection so that the system and service line are not left open to the atmosphere.

Does a self-sealing coupling prevent all SF₆ loss?

Not necessarily completely. A small dead volume containing gas can remain between the shut-off elements. SF₆ can also still remain in the connected hose.

What is dead volume?

Dead volume is additional internal volume in couplings, adapters, valves or lines that is filled with gas but is not part of the actual usable volume of the system.

Why is low dead volume important for SF₆?

The smaller the dead volume filled at the same pressure, the smaller the amount of gas that must additionally be handled during service.

Does pressure affect the amount of gas in the dead volume?

Yes. At the same temperature, the same volume contains correspondingly more gas at higher absolute pressure.

Is hose volume also dead volume?

In the broader service context, hose volume is also an additional gas volume outside the actual system gas compartment and must be taken into account during evacuation and recovery.

Why should an SF₆ hose not be unnecessarily long?

A longer hose has a larger internal volume. This means more volume must be evacuated and more SF₆ can remain in the line after servicing.

Does the service line have to be evacuated before filling?

If the ingress of air and moisture is to be prevented, the external service volume should be evacuated according to the intended service procedure before the gas compartment is opened.

What does DN 8 mean on an SF₆ coupling?

DN 8 designates the nominal size of the connection system. It is not identical to a specific thread size.

Which nominal sizes are available for WIKA SF₆ connection parts?

ICS Schneider offers, among others, WIKA connection parts in DN 6, DN 8 and DN 20.

When is DN 8 suitable?

DN 8 is suitable for many service, filling and measuring tasks, provided the required gas flow and existing connections are compatible.

When can DN 20 be useful?

For larger gas compartments or higher required gas flow rates, a larger nominal size can reduce service or gas recovery time.

Is the largest possible nominal size always better?

No. The nominal size should match the required gas flow. For smaller measuring and analysis tasks, large connection systems can create unnecessary service volume.

Can I adapt DN 20 to DN 8?

Yes, provided a pressure- and vacuum-resistant adapter suitable for the SF₆ system is used with the correct seals and connections.

Are several adapters in series problematic?

They can create additional sealing points, internal volume and mechanical length. A direct adaptation is therefore preferable.

Does an SF₆ coupling have to be vacuum-resistant?

If the service line or connected gas compartment is evacuated through the connection, the connection must also be suitable for the corresponding vacuum operation.

What pressure range do WIKA GCx connection parts have?

ICS specifies a range of -1 to +64 bar for GCx-06, GCx-08 and GCx-20.

What temperature range is permitted for the GCx connection parts?

ICS specifies an operating temperature range of -40 to +80 °C.

How leak-tight are WIKA GCx connections?

For GCV, GCA and corresponding measuring chamber versions, ICS specifies a leak rate of ≤ 1 · 10^-8 mbar · l/s. The components are tested for helium tightness.

Why is helium used for leak testing?

Due to its properties, helium is very suitable as a test gas for sensitive leak tests and allows very small leak rates to be detected.

What happens to SF₆ remaining in the hose after servicing?

In a low-emission service concept, this gas should, as far as technically intended, be recovered into a gas compartment, gas cylinder or via suitable service equipment.

Is it sufficient simply to unscrew the self-sealing coupling?

For a larger service volume that is still under pressure, the intended isolation and gas recovery procedure should first be carried out. The self-sealing coupling is the final part of the safe disconnection process.

Why does an SF₆ connection need a protective cap?

It protects the sealing surfaces and coupling mechanism from contamination, moisture and mechanical damage and, depending on the design, can provide an additional seal.

Which SF₆ coupling is suitable for a DN 8 hose?

WIKA GCx-08 connection parts are specifically designed for nominal size DN 8 and include self-sealing valves, couplings and adapters.

Where can I find the WIKA GCx-08 at ICS Schneider?

Further information is available under WIKA GCx-08 Connection Parts at ICS Schneider.

Which SF₆ hoses does ICS Schneider offer?

WIKA types GCH-08 and GCH-20 are designed for SF₆ service work in DN 8 and DN 20 respectively.

Do WIKA SF₆ hoses have self-sealing couplings?

Yes. The GCH hoses are equipped with self-sealing valves and couplings.

Does ICS Schneider offer a complete SF₆ filling set?

Yes. The WIKA GFK-10 is designed for filling and topping up SF₆ gas compartments from a gas cylinder.

Where can I find further SF₆ service components?

An overview is available under SF₆ Gas Solutions at ICS Schneider.