General Precautions

There are many issues to be considered when choosing products, we've listed a number of key below:

Stress Corrosion Cracking occurs in the simultaneous presence of sufficiently high tensile stress and a specific corrosive environment. For brasses this environment is usually one containing ammonia or a closely related substance. Atmospheres containing between 0.05% and 0.5% of sulphur dioxide by volume can also cause stress corrosion cracking. If ammonia is involved there may be a black staining surrounding the surface. The fracture surface may be stained or bright , according to whether the crack propagated slowly or rapidly.

Since all brasses are susceptible to stress corrosion cracking it is more important to control manufacturing, assembly, and operating conditions to avoid a combination of high stress and unfavourable environment that may cause stress corrosion.

Cottam & Preedy Ltd works with all our manufacturers to ensure our valves are less susceptible to stress corrosion cracking by using stress relief heat treatment. However dangerously high stress levels caused by careless fitting in assembly and installation can lead to stress corrosion cracking. Poor alignment, gaps at joints, over tightening and excessive use of jointing materials are examples of bad practice in this respect.

Screwing taper-threaded connections into parallel threaded brass valves or vice-versa is another example. Where jointing materials, jointing compounds, or a combination of the two, are used to seal the thread, it is all too easy to overtighten the joints to a point where a very high circumferential hoop stress is generated in the female member. The swaging of the end of the valve is one indication that it is being over tightened.

The pressure temperature rating of a valve is determined not only by the valve body material, but also by the sealing material of the seat, stem packing, and body seal. Choice of sealing materials should always be based on all relevant factors, which may include the service media, service temperature, service pressure, media velocity within the line, frequency of operation and the size of the valve.

When the valve is subjected to extreme variation in ambient temperature, fluid thermal expansion may cause the ball to exceed intended operating pressure. This condition tends to be more severe when the ball is in the closed position for a long period of time during extreme temperature variation.

Generally valves are designed to equalise the pressure within the ball cavities but the user must take measures to ensure that the valve does not exceed its rated pressure due to fluid thermal expansion.

Please contact our technical department on 0121 552 5281 or email enquiries@cottamandpreedy.co.uk