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In order to reduce such emissions, it is important to develop a valve testing system that allows the most common stem seals to be tested and compared.

Potential leaks usually occur at so-called source points: seals either in or between pipes and/or vessels. Valves are an example of this. A valve consists of an upper part and a valve body with an orifice. By means of a spindle, a valve is manually or mechanically moved to position 'open' and 'position closed', respectively This means that the product flow is proportional to the size of the valve opening.

An estimated 60% of unwanted emissions are due to valves, and are mainly caused by decreasing by loss of compression. A compressible stem seal seals when a certain surface pressure is applied to it by means of a gland bush. The stem seal is thereby pressed radially but deflects in an axial direction, and this movement causes it to seal.

So the question is, why does this compression rate decrease?

To answer this, endurance tests were carried out by ITIS in Goes The Netherlands. Director Colin Zegers explains the choice for that specific location.

“We have test bunkers here in which, among other things, valves and sealing materials can be tested for leaks. This testing is done using the so-called sniffer-pressure method in accordance with internationally recognised standards.”
A compression stem packing of the valve could contain five or six rings of gaskets, all of which are normally fitted and pressed on at the same time. The top gasket ring then appears to perform its sealing role well, but the same is not true, or is less true, for the remaining rings.

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Examples of stem seal leakage - Source: ITIS

“The idea is that each ring is basically compressed by 30% before the next one is installed. That way you get an optimal distribution of forces across rings. If you only pre-compress some of the rings, which is obviously a lot faster, then they have to do all the work; the rest is just 'padding'. The less the precompression, the harder the packing rings have to be tightened, and then, due to deformation of the rings, at some point it has to stop”, Zegers explains.

The tests are always conducted under the same pressure and temperature conditions. The stem seals are assembled in the same way and tightened with the same force. By placing special pressure sensors on the valve, the force required to tighten gasket pusher bolts can be measured, ensuring optimal sealing action.

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Examples of stem seal leakage - Source: ITIS

“By mounting so-called smart positioners on the control valves, it becomes possible to measure the number and size of the valve movements, often in combination with the pressures and temperatures of the process. To make the whole thing even 'smarter', insofar as possible, sensors for measuring and predicting emissions are linked to the positioners already in place.”

It has been established empirically that the force of tightening the gland bush, the type of stem seal and the number of turns are interrelated. Furthermore, a gasket is found to relax immediately after tightening - decrease in pre-pressure - resulting in loss of bolt tension. This has a negative impact on the leak proofing of the valve. Several factors influence this.

“Consider the make and type of stem seal, alignment, tolerances, roughness and temperature. After relaxation, the axial force is too low, the upper ring is tightened, but cannot tolerate this and deforms, resulting in a (partial) drop in surface pressure and leakage reoccurs. The reason for this relaxation has not been established at this time; further research will have to be done into this.”

When choosing a particular brand and type of stem seal, factors such as pressure, temperature, type of gasket material in relation to the product and the maximum allowable leakage value must all be considered. The tests carried out show that dynamic valve seals leak under certain conditions, despite the fact that the process conditions specified by the manufacturer are met.

Figure: Sources of fugitive emissions in the process industry (source: European Sealing Association)

Often, such values are not or not sufficiently substantiated by independent tests. What also does not help is the fact that there is as yet no standard or regulation for stem seals. In his role as team leader of the project, Zegers is currently writing a standard procedure for the best stem seal testing method.

The aim is to standardise as much as possible per stem seal type and material, the method of assembly and the required compression. It is also important to unambiguously standardise the relationship between the force of tightening the pusher, the type of gasket and the number of tightening torques, so that everyone starts working in a uniform way and the results of those findings are interchangeable.

“Currently, the claims made by manufacturers of stem seals are uncommitted and not verified against a standard. Fortunately, the ESA and many other stakeholders agree we need standardisation. In this way test reports will lead into increased
product responsibility,” Zegers says.

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Interreg project Circular Maintenance

In this project, several public and private partners are working together, led by the (Dutch) Knowledge and Innovation Centre Maintenance Process Industry (KicMPi) and the Belgian Maintenance Association (BEMAS), to reduce the material footprint of the process industry in the south of the Netherlands and Flanders, while at the same time achieving economic benefits. The project is supported by Interreg Flanders-Netherlands, a European program that finances cross-border projects for smart, green and inclusive growth. It is funded from the European Regional Development Fund (ERDF).

The project has been delayed because of corona but more significantly because of its complexity. "As the project ends at the end of this year, we are looking into whether we can continue the studies and if so, in what way. Given the many thousands of tonnes of fugitive emissions annually, this is by no means an unnecessary luxury," says Zegers.

A scanning system that tracks on-site valves that start showing leakage behaviour in the near future would therefore be ideal. In that regard, Zegers has good news.

"Currently, a prototype test system is being built and tested in-house at ITIS as a kind of spin-off from the Interreg project. Unfortunately, due to the patent applications, I cannot and may not say more about that at the moment."
The Interreg project officially ends on 31 December 2022. During the closing event on 15 November, the results achieved will be shown and preliminary conclusions shared. To learn more visit   www.circulaironderhoud.eu   .

Text: Wouter Oude Groothuis
Images: Colin Zegers ITIS BV, SHUTTERSTOCK