Does your boiler and fittings comply with sub-section 40 of the 1955 Factories Act?

09 March 2011

The 1955 Factories Act Sub Section 40 states "a boiler and all its fittings must be properly maintained" and yet HSOs are frequently called out to accidents where calorifiers, boilers and hot water heaters have exploded because safety valves, encrusted with limescale, have not operated correctly. If there is a build-up of pressure in the vessel and safety valves are not operating, explosion will occur causing severe damage and possible injury to personnel.

Boilers come in a vast array of configurations, however, all operate under the premise that water will boil at higher temperatures when under pressure. Operating at a higher temperature allows more energy to be put into the steam in the form of higher pressure. Should the integrity of a boiler be compromised, either from exceeding the design limits or because of mechanical failure of the units, the results are catastrophic. The water in a boiler is in a liquid state only because it is under pressure. As soon as the pressure is removed the water will "flash" into steam in adiabatic reaction. This steam, while at a lower pressure, will expand at an exponential rate, thus the fragments of an exploding boiler are propelled at horrifying speed.

For example, a boiler having a one cubic foot volume, operating at a pressure of 150psi, has roughly 21,600 pound feet of energy contained in it. To put this in perspective, if the boiler weighs 100lb and develops a hole that vents its contents in a tenth of a second, the boiler would be accelerated to a velocity of 118 feet per second, approximately 80 miles per hour! A similar scenario would occur if the safety valve failed, resulting in catastrophic boiler failure.

In March 2001 a calorifier used to provide domestic hot water ruptured during operation in a factory. Consisting of a copper shell, bronze tube plate and copper "U" tube bundle, the old waterside shell was fitted with a relief valve. The tube nest was supplied with steam controlled by a thermostatic temperature controller. The shell longitudinal seam and the end caps were riveted.

The system had been out of service for an extended period and had been scheduled for demolition for the past five years. Because it was "about to be scrapped", it had not been descaled. Following a requirement for extra hot water, the vessel was returned to service and shortly afterwards ruptured under pressure. The vessel tore along the riveted longitudinal seam and around both end caps. The force of the explosion moved the vessel about two metres off its foundations and severely damaged adjacent equipment and parts of the building in which it was located.

The inspectors' report noted that the bends at the ends of the tube bundle had what appeared to be long-term minor cracking and evidence of more recent sudden cracking. In conclusion he surmised that this more recent cracking caused an overpressure that could not be handled by the relief valve, (especially in what appeared to be its impaired condition resulting from a long-term build-up of scale around the safety valve seat and disc) and the vessel ruptured. A later report concluded that the disc and valve seat could not be separated, because of scale build up, and the valve did not lift under a test pressure of 175 psi.

A further illustration of the dangers of scale build-up in safety valves is well documented in an explosion of a calorifier in a modern single storey office building. Fortunately, in this instance the calorifier exploded at 2.00 a.m. so no one was injured. The calorifier rocketed through the reinforced concrete floor of the building and through the roof. The blast was of sufficient velocity to destroy a large section of the floor and resulted in considerable damage to the building, computer equipment, plant in the basement and to a telephone exchange.

The calorifier had ripped apart at the bottom end, at or near the shell to endplate seam. The shell minus its bottom had then landed on the roof of the building and the bottom endplate was found under the rubble, still resting on its concrete supports. It was estimated that an explosion force equivalent to approximately 23lb of TNT had been created to cause such damage. It was established that the calorifier had been out of use for a period and the flow and return valves on the shell had been in the closed position. On the day of the accident, steam had been leaking into the coils and over a period of 18 hours the temperature, and consequently the pressure of the 600 gallons of water in the calorifier, gradually increased until finally the vessel exploded at what was later calculated to be a pressure of approximately 65 psi compared to its normal pressure of 23 psi.

A vent pipe had been fitted to the system, but this was isolated when all valves on the calorifier shell were closed. This only left the safety valve to relieve the pressure situation, but it failed as scale had accumulated around the valve head and its seat.

Following the accident, laboratory tests were carried out to simulate conditions where a safety valve could be prevented from lifting at its set pressure due to adhesion between the valve head and its seat. Experiments were conducted on a variety of valves at different temperatures. Water obtained from the site mains was dripped into safety valves and allowed to evaporate. Scale was thus formed and, on one occasion, a valve, which had been set and tested at 35 psi, was scaled to its seat after only 40 days. Under subsequent hydraulic test the safety valve operated with a loud report at a pressure of 92 psi.

The inspectors concluded that it was likely that the safety valve had "wept", which in the warm environment of the plant room would have produced the scale, which had built up in just a few months.

Following their findings, the inspectors conducted trials on other safety valves taken from heating boilers and systems in different parts of the country. Of the total number of valves checked, approximately 2% showed significant sticking.

So how can such accidents be avoided?

Regular testing of safety valves is essential and, if accidents are to be avoided, it is imperative that ongoing scale prevention is carried out on all hot water boilers and calorifiers. One way of preventing scale build-up, not only on safety valves, but also in boilers, calorifiers and pipelines, is through the installation of physical water conditioning equipment.

The major advantages of using, in particular, "fit and forget" electronic scale control equipment such as the Patented Scalewatcher ENiGMA are:

It requires no maintenance, plumbing or chemicals
It removes existing scale within the water system
It protects the entire water system including safety valves
The quality of water remains unchanged
It reduces bacteria levels (scale is a prime breeding ground for Legionella)
It increases the life of capital equipment including boilers and calorifiers
It can be fitted to any pipe material and size ranging from 5mm to 1,000mm

Scalewatcher has invested significant sums in the research of electronic systems in the USA, Europe and Asia. The companys' research has led to the implementation of an effective scale deposit-control strategy, directly reducing energy costs and contributing to customers' profits, especially prudent considering the implementation of Climate Change Levy.

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Does your boiler and fittings comply with sub-section 40 of the 1955 Factories Act?

09 March 2011

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