Frequently Asked Questions

Hydrate Formation

Hydrates do not normally form within the SJP because of high velocity of flow and the combination of HP and LP flow. If the predicted outlet temperature is within the hydrate formation band, then injection of hydrate inhibitor and / or use of heat tracing are recommended. It is usually recommended not to run the HP flow alone (for over a few minutes) without flowing the LP gas in advance.

Typical Size

All SJPs are custom designed and built to meet customers’ individual requirements.

Thus, there is no specific limit to size in terms of diameter or length. Based on experience, lengths have varied from 0.3m to 8.0m and the SJP discharge diameter from 1/2” to 36” NB.

What Material?

SJPs can be fabricated from any machinable or weldable material. Typical materials are Low Temp Carbon Steel, Stainless Steel, Duplex Stainless Steel and Super Duplex Stainless Steel. Materials can be NACE certified for sour applications. The internal surfaces can be coated with hard wearing materials such as tungsten carbide, Stellite, etc. or lined with ceramics for erosive duties.

Pressure Rating

All SJPs are custom designed to form an integral part of the customer’s piping system, thus the pressure / temperature rating of the SJPs will match that of the interconnecting pipework.


Flanged connections, rated from class 150# to 2500#. For higher pressure ratings, API 6A, or API 17D for subsea applications, are supplied. Compact type flanges or clamp type connections are also possible.

Pressure Relief (PRV) Required?

In most cases the SJPs and Silencers are designed to meet the highest operating pressure, plus an allowance for safety. Normally no pressure relief is needed. It is, however, possible to incorporate a Pressure Relief Valve (PRV) if the LP side piping system is of a lower pressure rating than the HP side piping, or to meet site specific requirements such as the site fire case.


Noise applies to gas SJPs only. The produced noise level may often exceed 85dBA.

Silencers can be installed on the LP inlet and the discharge line of the SJP to prevent noise travelling through the interconnecting pipe work. In cases where noise emitted directly through the body of the SJP exceeds the acceptable limit, the entire SJP can be lagged with acoustic insulation materials. The normal acceptable limit is generally 85dBA at 1.0m from the SJP. There are, however, exceptional cases where lower noise level is required.


If vibration from the SJP is predicted to be a problem, then the SJP and Silencer supports can be fitted with custom designed anti-vibration mounts.

Temperature Drop Effect

This is an issue dependent on factors such as HP and LP pressure, temperature and flow rate and the presence of liquids in the LP flow. In general, the theoretical Joule Thomson temperature drop across the SJP does not fully materialise, mainly as a result of the LP flow mixing rapidly with HP gas and the pressure increasing along the mixing tube and diffuser. Low temperature may be experienced if only HP gas passes through the SJP with no LP gas.

In general, the outlet temperature is often lower than the LP or HP temperature by approximately 20% to 30%, whichever has the lower value.


Pressure and temperature instruments are recommended for monitoring the performance of the SJP only. The incorporation of a flow measuring instrument in the HP line would allow wear in the motive nozzle to be detected, if required under erosive conditions.

Changes to Operating Conditions

Changes to the operating conditions will affect the performance of the SJP. The SJP normally passively responds to such changes by adjusting the LP pressure at its inlet. Its operating range can be extended by simple control and changes of its internals.

Change-out of Internals

There are two internal components within an SJP, namely the HP nozzle, and diffuser or mixing tube unit. The internal geometry of these two items is what gives the SJP its operating characteristics. Change-out of the internals is only required if there is a significant change to one or more key operating parameters. As the nozzle and diffuser are matched to each other, it is usual to change-out both internals together, unless the change is primarily related to the HP flow. Changing the internals is relatively simple and can be carried out within a few hours; the time needed includes full isolation of the system, venting and draining before the SJP can be opened. Detailed procedures related to SJP internals change-out are available for this operation.


Normally no active control is needed on production boosting or pressure boosting applications. In other applications, control of the LP or Discharge pressure would be important, e.g. in flare gas recovery applications, control of the LP pressure is generally required to prevent process upsets or to protect the LP separator from exposure to below atmospheric pressure.

Start-up / Shut down

The start-up / shut down procedure for the SJP is very simple and easy. Detailed procedures are provided for each application to ensure all site related and HAZOP issues are considered.

Can HP gas boost LP liquid pressure?

Not directly via the SJP, however see Sections 5, 6 & 9. HP gas can only boost LP gas.

However HP liquid (oil or water) can boost LP gas pressure.

Liquid in Gas Limitations

Presence of liquids in the HP or LP gas is tolerated when the liquid rate is less than 1% to 2% of the gas f low rate at the operating P&T (i.e. at GVF greater than 98%).

Beyond these values separation of the liquid phase is recommended prior to entry to the SJP as it begins to significantly affect the performance of the SJP. Transient conditions, such as those at start-up when liquids may be swept through pipelines or loaded wells, are acceptable as a temporary condition.

Liquid Slugs

Gas SJPs are tolerant of liquid slugs passing through them. Liquid slugs will not mechanically damage the SJP or Silencers. However, performance will be temporarily affected until the slug has passed through.

Effects of Turn Down

SJPs offer stable LP flow rate turn-down from the design flow rate to near zero LP flow rate without the need of any active control. Hence they are tolerant to sudden changes in LP flow rates. If the HP pressure / flow rate is significantly reduced below the design value, the nozzle needs changing to match the new HP condition. If the discharge pressure changes, the SJP automatically adjusts the LP pressure to match its performance.

An SJP can be operated down to zero suction flow rate without causing mechanical damage. However, at zero LP flow conditions and full HP flow only, the temperature effect and possibility of hydrate formation needs to be investigated.

Protection of LP Separators

There are a number of ways to protect the LP separators from under pressure. One method is to install a bypass line equipped with a control valve which routes fluid from the outlet of the SJP back to its LP inlet. This valve will allow recirculation of some gas and will prevent the LP pressure falling below a set point. There may be other viable control strategies.


Peak thermodynamic efficiency of an individual SJP device is only around 30%. However, this should not be the basis for selecting or rejecting these devices because they often use energy which was previously being wasted to do useful work. In some cases the application of SJP can bring dead or shut-in wells back to life. Furthermore, because of the way in which they operate, SJPs have significantly lower maintenance and running costs.


Not a major issue unless sand is produced. In this case, the internals can be coated with materials resistant to erosion or lined with ceramic materials.

Installation Orientation

SJPs can be installed and operated satisfactorily in any orientation or arrangement. For gas or liquid driven SJPs installed vertically, there would be a preference for the flow to be arranged vertically downwards, as that provides the best orientation for liquid drainage. The most appropriate orientation is primarily dictated by site conditions.

Installed distance between SJPs and Silencers

Ideally Silencers should be connected directly to the LP inlet and the discharge outlet of the SJP. If this is not possible, any pipework between the Silencers and the SJP should be acoustically lagged to limit noise emission through these sections.

Distance from the SJP to HP or LP source

This depends highly on the site conditions and constraints. It is normally recommended that SJPs are located as close as possible to the LP source to minimise LP piping pressure loss. Site conditions may, however, dictate otherwise.

Predict / guarantee production increase

Production increase as a result of reducing the back pressure on wells depends on the Productivity Index (PI) of the well, which is outside the control of the SJP. If the PI of the candidate wells are not known, the wells could be tested via the test separator to determine their suitability and production characteristics.

Can an SJP revive dead wells or liquid loaded wells?

Yes. There has been a number of successful cases where the SJP has revived dead wells or liquid loaded wells.

However, there are cases where the limited HP pressure and flow rate available is insufficient to achieve the initial revival or unloading. In these cases, other techniques should be used for the initial revival of the well before the SJP is brought on-line to maintain steady production.

Are there any upstream / downstream straight pipe length requirements for installing an SJP?

There are no critical upstream / downstream straight pipe length requirements for SJP installation. However, sufficient lengths should be allowed with break-in flanges to allow SJP internals withdrawals. This is needed mainly on the discharge and HP sides.