Addressing competence in silo servicing and maintenance

Over-pressurisation of storage silos during powder deliveries is a significant safety issue, one which the cement and aggregates industries (amongst others) have become cognisant of over the last decade. Whilst it is a complex problem, the risks can be effectively contained and neutralised by ensuring a few key criteria are met – amongst them, the establishment of a scheduled maintenance programme, carried out by a qualified and competent engineer. However, as silo protection experts Hycontrol have observed and experienced, this essential competence level is frequently lacking, due in the main to an absence of appropriate training. Here, Managing Director Nigel Allen discusses these shortcomings and outlines some of the basics every person responsible for silo pressure safety needs to know.

Competence (or the lack thereof) in servicing and maintenance is a serious if somewhat unspoken issue currently facing the cement and aggregates industry. This is particularly true when it comes to silo protection and pressure safety equipment maintenance, an area which has historically been overlooked or neglected. Even today, as awareness of the issues increases in the industry, there are problems.

With a recognised shortage of skilled engineers in the country at present, it is unsurprising that there are shortcomings in this area. But lack of competence in servicing, particularly when dealing with crucial safety equipment, can actually exacerbate onsite risks.


It is an uncontroversial Health & Safety expectation that a competent person – in other words, a person with both appropriate practical and theoretical expertise – should be tasked with regularly inspecting work equipment to ensure it is functioning and safe.

The purpose of this inspection should be to highlight any issues which are or may develop into a safety risk. The more hazardous the environment the equipment is in or the more important it is for maintaining site safety, the more frequently and thoroughly it should be checked. This inspection should be done on a routine, scheduled basis to ensure that nothing is overlooked.

On paper, this seems pretty straightforward. However, as some have observed[1] there can be a disparity between corporate safety policies and initiatives and the day-to-day realities of maintaining site safety. Executives may talk about the need to achieve world-class standards but don’t have understanding of specific applications, leading to pressure on workforce members who may require resources or support which are simply not being made available. This can be compounded by issues of cost or a lack of clarity about the problems that are faced.

When it comes to the specific issue we are considering – competence in servicing and maintaining silo safety equipment – we have to start with pretty fundamental questions: How does one judge if the person tasked with the upkeep is competent at all? Given the nuances and complexities of silo pressure safety, by what standard is competence judged? Does this mean that safety standards are really being adhered to?


Silos for the storage of powdered products are commonplace in a wide range of industries, including cement, quarrying and many others. These vessels can be anything up to 50 metres (164 ft.) or more in height, and require a number of essential safety devices to be mounted on top, such as the Pressure Relief Valve (PRV), pressure sensor, high-level alarm and the air venting dust filtration unit. Normally, this safety equipment will be connected to a control panel at ground level, and this set-up is collectively referred to as a silo protection system.

Many experienced industry engineers and service personnel still believe that the main risk to silos comes from overfilling. This is in fact not the case. Whilst it is true that overfilling can be a significant problem, particularly if an excessively high product level results in damage or ‘blinding’ of the filtration system, the real danger comes from the risk of over-pressurising a silo.

The first thing to understand is that silo over-pressurisation is a problem arising from the pneumatic delivery of product from a tanker into a silo. If the air used to convey the product can enter the silo and exit via its filter venting unit without restriction then there will be no over-pressurisation issues. Over-pressurisation only ever occurs when the volume of air entering the silo exceeds the volume of air able to escape. There are two likely causes for this: either the airflow out of the filter unit is restricted, or the airflow into the silo is greater than the system is capable of exhausting. As most silos are not pressure-rated vessels, it only takes a small increase in pressure (as little as 1 psi) to over-pressurise the vessel.


The dangers and possible consequences resulting from insufficient silo protection include:

  • Danger to staff: If a silo over-pressurises, it could rupture, or blow the filter housing off the top to fall into the yard below. Site personnel, contractors and drivers are at risk of severe injury or even death as a result.
  • Damage to silo: Rupture of a silo will resulting in extensive disruption and loss of production. Sites will be faced with either expensive repairs or having to replace the entire silo.
  • Emissions into the atmosphere: Product blown out of a silo, either via the filter, pressure relief valve or even as a result of the vessel rupturing, will likely cause environmental damage. This can result in large fines, expensive clean-ups and long-term damage to company and site reputation.

The fact that the vital safety equipment is mounted at height creates a whole new set of problems. Any safety equipment is only ever as good as the last time it was tested, and best practice guidelines state that silo pressure safety equipment must be tested prior to every pneumatic filling operation. In other words, a member of staff has traditionally been required to climb the silo to manually test the equipment, regardless of weather conditions or visibility. It is plain to see that this is a risky undertaking.

Slips, trips and falls continue to pose significant workplace safety issues. Should they take place at height then the danger is significant – for example, in the UK, statistics from the Health & Safety Executive (HSE) show that falls from height account for 26% of all fatal injuries in the workplace[2]. Silos are combine all the notable dangers of working at height: besides their sheer height and difficulties accessing the top, they are usually outdoors, putting workers at risk from slippery surfaces, high winds, and other harsh weather conditions.

HSE guidance also strongly advises against allowing anyone lacking the necessary skills to carry out work at height. This is an important consideration, and brings us back to the central concern of this article – do the people being sent to the silo-top actually know what they are doing when they get there? Will they be able to fully test the safety equipment and verify its functionality, or will they only be able to visually inspect and clean it? Do they really know what problems or warning signs they are looking for? If the answer to any of these questions is no, then staff members are being put at risk by climbing silos for no achievable ends – and their colleagues on the ground are at risk from potential silo protection failures.

So, there remains a basic issue of competence in silo safety equipment servicing which, if it is lacking, can undo any progress that has been made solving the other issues outlined previously. But what can be done to address this?


The preceding sections give a basic sketch of a significant, multi-faceted and widespread problem. Knowledge of these facts should be commonplace; however, this does not appear to be the case. Misconceptions like ‘a regularly-opening PRV is functioning correctly’ or ‘the biggest danger to silos is overfilling’ still abound.

So, the need for service personnel to be trained and experienced in essential silo pressure safety is clear. Indeed, it is advisable for any staff members who regularly find themselves working in close proximity to silos to be at least alert to the warning signs of problems arising from over-pressurisation.

To this end, Hycontrol has made a variety of resources available to help improve safety and education around this topic. Having worked closely with the quarry and aggregates sectors for over thirty years, Hycontrol has become one of the most experienced companies to specify and supply level and pressure equipment in the UK. A website, has been set up as an online resource for videos, photographs, and other materials to help raise awareness of the risks of silo over-pressurisation and the warning signs to look out for.

Whilst the website offers a useful introduction, it has been recognised that more intensive, structured training is required for maintenance staff tasked with silo safety, as well as for those responsible for site safety and or deliveries. Given the level of risk involved, Hycontrol has now collaborated with the Mineral Products Qualifications Council (MPQC) to establish a series of short training courses, intended to provide essential safety awareness of the following key elements to both front-line staff and managers:

  • How a delivery works
  • Consequences of failure
  • Dynamics of delivery with regards to flow and pressure
  • Essential equipment required
  • Maintenance procedures and best practice

All of the above are basic principles and facts that anyone involved in the pneumatic delivery of product from tankers into silos should be aware of. The purpose of this MPQC-approved training is to raise the level of awareness and quality of practice in the industry with regard to silo pressure safety. By getting the industry to acknowledge and talk about this problem, and by sharing knowledge and best practice, it is reasonable to expect that standards will start to will improve – and with them, we will see a reduction in the number of pressure incidents.


Silo over-pressurisation and the problems stemming from it remain a serious issue globally. Whilst there have been some improvements, awareness of the issue and its causes remains challengingly low. For many managers and executives who have direct responsibility for silo safety, the problem remains frustratingly obscure and is not given sufficient priority. Site staff and service personnel are not receiving sufficient resources and training to address the issues. The resulting lack of competent servicing will inevitably exacerbate existing problems, potentially leading to disaster.

Correcting this trend towards entropy will require a committed effort from all concerned parties, but it is achievable. There must be raised awareness of both the causal issues and the best practice methods to negate them; an understanding of the silo protection application at the boardroom level and an appreciation of safety goals on the front line; and sufficient provision of both maintenance training and technical resources. If industry makes these goals a priority, then improvements in safety standards will follow.