March 02, 2015

02/03/2015: Silo safety - A comprehensive overview on silo safety guidelines

by Portasilo, UK

first published in Milling and Grain, January 2015

Silos that are filled directly from a pressurised tanker need to incorporate suitable mechanisms to allow displaced air within the silo to escape during filling. If these mechanisms are insufficient for the pressures created within the silo, or if the equipment is poorly maintained and becomes inefficient, dangerously high pressures can build up within the silo. 
This can lead to a catastrophic blow out, damaging plant and equipment, dispersing possibly toxic silo contents over a wide area and endangering the lives of employees.
In this report, we bring together all of the guidance and recommendations into a single, easy-to-understand guide to safeguarding your plant against silo over-pressurisation. At the same time, we highlight the implications of the HSE and Defra guidance for silo safety equipment.

Prepare for peak airflow

One of the major changes to existing assumptions outlined in the HSE document was the need to specify silo safety equipment to cope with a peak airflow from the silo of 13,000m³/hour. The previous assumption had been that peak airflow caused by over-pressurisation during tanker filling could only reach around 2000m³/hour.
To test the new assumption, Portasilo built its own test rig and modelled a variety of over-pressurisation incidents. The company found that it was possible to reach peak airflow from the silos of almost 13,000m³/hour as a result of over-pressurisation. The next step was to investigate the ability of existing safety equipment to cope with this increased airflow and, where necessary, redesign it. 

Silo filters – the first line of defence
All silos that receive pressurised tanker deliveries must be fitted with a filter in the roof. This allows displaced air from within the silo to escape to the atmosphere. Portasilo tests found that filter sizes and specifications did not need to be changed in response to the new peak airflow assumptions in the HSE guidelines. If properly sized and maintained, existing filters could already cope with an airflow of 13,000m³/hour and it was our assumed minimum filter velocity that had changed not the requirement for more filter area.

However, regular filter maintenance is essential to sustain the required levels of performance. Silo filters incorporate filter bags to prevent dust escaping into the atmosphere. These bags must be regularly cleaned to prevent clogging which could inhibit the free outflow of air from the silo.

A number of automated filter cleaning mechanisms are available, but the most efficient and effective are reverse jet-cleaned filters. These systems use jets of air to blow the dust from inside the filter bags. It’s also important that the air supply to the filter is clean and dry. Moist air can quickly exacerbate the clogging of filter bags, particularly with particulate powder silos. 

Pressure relief valves – the last line of defence
If pressures within a silo build up to such an extent during filling that the filter cannot release enough air to maintain silo pressure within safe limits, the pressure relief valve (PRV) is triggered. This valve is designed to open below the design pressure of the silo.

Each pressure relief valve is designed to open at a particular ‘set pressure’. 

PRVs contain a plate, which lifts up when pressure reaches the ‘set pressure’, releasing air to the atmosphere and immediately reducing pressure within the silo. The plate is held in place either by springs or by a dead weight, depending on the design. 

Implications of the new guidelines for PRVs
Given the new guidelines for a maximum 13,000m³/hour airflow from the tanker, it is essential that PRVs are sized for this maximum potential out-breathing. Many PRVs on the market at the time the guidelines were released were not large enough to allow this volume of air to pass through, potentially allowing dangerous pressures to build up inside the silo.

Investigations also found that some spring-type valves could become coil-bound and weren’t opening properly. This could cause dangerous pressure build-ups in the silo. When tested, such coil-bound valves caused air to accumulate within the silo by preventing it from escaping at the required rate. The silo’s design pressure is quickly exceeded in these conditions, with potentially catastrophic results. 

Choosing an effective PRV

When choosing a PRV you should look for a model that has been tested and certified for a specific volumetric capacity and set pressure. The volumetric capacity of the valve should be 13,000m³/hour to meet HSE guidelines and you should choose a PRV with a set pressure slightly lower than the design pressure of your silos.

Some PRVs are now 350mm in diameter, to cope with the greater volumetric capacity requirements – many were previously 250mm across. You should also choose a valve with long travel springs that will not become coil bound over time, reducing the effectiveness of the valve. Some PRV manufacturers use the same spring specification for all sizes of valve and adjust them to open at higher pressures. 

This can lead to inconsistencies in performance, especially if end users at the plant later adjust the springs. A more reliable system is to use different specifications of spring for each PRV specification. It means there is no room for human error in spring tightening, as the spring properties are set at manufacture and cannot be adjusted by the end user.
It is good practice to choose a PRV with a set pressure below the design pressure of your silo, but not so close that even the slightest PRV inefficiency or delay would cause the silo design pressure to be exceeded. 

Another problem with some PRV designs is that the weather-proof cover can impede air flow through the valve. In testing, the valve works perfectly well without a cover, but once a cover is fitted, it’s shape can inhibit the free flow of air. Try to look for a PRV that has an aerodynamic cover that flares out at the edges, allowing air to pass freely. 

As well as allowing air to escape from the silo, PRVs also help to relieve under-pressure or vacuum situations within the silo by allowing air to flow in from the outside. Tests of older Dead Weight type PRVs found that the inflow mechanism was often susceptibleto dust clogging, making them inefficient. Once again, it’s important to choose tested and certified PRVs, preferably with an inflow capacity of 5000m³/hour.

Auto shut-off – your early warning system
PRVs should always be considered a last line of defence. Ideally, pressure build-ups should be alleviated before the PRV is triggered. That’s because when the PRV opens, it will emit dust from the silo over a wide area, with potentially harmful consequences for human health and the environment. 

Automated warning and shut-off systems can be installed to alert operatives that dangerous pressure levels are being reached within the silo, before the PRV is triggered. These systems require a probe to be fitted at a high level within the silo to monitor pressure levels. When pressure approaches a pre-set level, which should be just below the set pressure for the PRV, an alarm sounds to alert the operative filling the silo, enabling them to reduce the pressure and stop filling. If pressure in the silo continues to rise, the auto shut-off valve will be triggered, blocking the inlet pipe and preventing any more powder from entering the silo. 

Choosing an auto shut-off system
It’s advisable to choose an auto shut-off system with a two-stage alarm, which will give operatives a little more time to reduce the filling pressure, before the inlet pipe is automatically shut off. The first alarm is sounded as pressure reaches a set level, warning the operator that they need to reduce the pressure. If they fail to do so and pressure remains the same or rises, a second alarm will sound, giving the operator 20 seconds to reduce inlet pressure before the inlet pipe is shut off. These automated shut-off systems are now required on all new silos.

Making sense of maintenance guidelines
The HSE and Defra guidelines set out recommended maintenance regimes for silo safety equipment. It’s clear from past experience that poorly maintained filters and PRVs are a major cause of silo over-pressurisation incidents. If this equipment is working properly, along with associated auto shut-off systems, there should be no need for plants to worry about silo over-pressurisation.

The HSE recommends that valves should be tested “routinely” to ensure they have not seized up. Defra recommends that the valve seating is tested weekly. Both require filters to be checked weekly – or three-monthly if remote-monitoring systems are fitted. All of these guidelines mean that it may be necessary for an operative to climb to the silo roof as often as once a week to carry out the necessary checks.
However, this requirement conflicts with the requirements of the Work at Height Regulations, which were introduced in 2005.These regulations do not apply if a permanent staircase is fitted to your silo to provide access to the roof. However, these can be costly and, in most cases, silos are simply fitted with have a vertical ladder for roof access. As such, they are governed by the Work at Height Regulations. 
These regulations state that the need to work at height should be avoided wherever possible. Where working at height cannot be avoided, measures should be taken or equipment installed to prevent falls. If the risk of a fall cannot be eliminated, then equipment should be installed or measures taken to minimise the distance and consequences of any fall.

Ground-level testing – a solution to work at height restrictions

One simple way to avoid the need to visit the silo roof on a regular basis is to fit ground-level testing systems to your silos. PRVs can be fitted with ground-level test facilities, and self-testing, self-cleaning pressure monitors can be installed in your silo to ensure that high-level probes are working correctly. All of your silo safety equipment can then be tested from ground level from an easy-to-use console fitted to the side of the silo.

A ground-level testing regime should then be enforced for all filling operations. Before filling can commence from a pressurised tanker, the operative should have to carry out a series of ground-level tests to ensure all silo safety equipment is working correctly. Only if all ground-level tests are positive will the inlet valve be opened allowing the silo to be filled safely.

Read the magazine HERE.

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine GFMT
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