ASK Piearcey


Pressure and Flow Control in Multiple Extract Point Systems
April 25, 2018, 6:28 pm
Filed under: Uncategorized

The purpose of this article is to describe extraction ductwork design approaches that we have seen in practice and try to clarify potential problems with some of these approaches and show that there may be better alternative ways of optimising and extract system.

Large, multi-operational processes (e.g. a sewage works, a chemical manufacturing plant or a Food production facility) will typically have several emission points.

Sometimes the emission sources are of an intermittent nature and there is a need to automatically balance the system to maintain the desired extraction from other sources.

We have often seen the deployment of a pressure control loop to handle this type of application. Here the speed of the main fan is controlled to maintain the system pressure at a particular confluence and in doing so there is some control on the conditions for the extraction system.  However, there are, as always, good and bad engineering practices involved in the design of such systems.

Fig 1 below shows a typical manifold for a multiple point extraction system.

Fig 1 pressure control

We have actually seen this format in operation and needless to say it is not effective. The emission sources are on a manifold and are dependent on the flow from each of the other sources. The static pressure at the end source will vary by the number of combinations of the other sources and hence it is futile balancing that emission source to a single available pressure.

Fig 2 shows a better engineered approach whereby each extract is independent to a collection plenum which is then subject to pressure control. Here each leg is individually balanced to the target static pressure at the plenum. If the fan has the capacity to maintain the static pressure at the plenum then the system can remain automatically balanced. Note that this approach would also favourably respond to variable pressures upstream of the fan. If there was an adsorber as an abatement system, then if that adsorber became blocked then the fan would find it more difficult to reach the target negative pressure in the plenum and would speed up accordingly.
We have actually seen this format in operation and needless to say it is not effective. The emission sources are on a manifold and are dependent on the flow from each of the other sources. The static pressure at the end source will vary by the number of combinations of the other sources and hence it is futile balancing that emission source to a single available pressure.

There is however a risk with this approach. Imagine if one or more of the extracts is critical and it is possible to block the duct with solids or liquids. Any blockage in the extract would cause the fan to slow down thus exacerbating the problem. Fig 2 represents a viable solution where there is no significant risk of blockage or unwanted line closure.

What happens if you move the pressure transducer/ transmitter unit downstream of the fan?

If we were to look at what might be downstream of the fan (e.g. abatement) then it may be desirable to control the fan to respond to pressure variation down-stream of the fan.
Fig 3 pressure control

In fig 3 the pressure sensor is now up stream of a carbon adsorber and downstream of the fan. If the carbon adsorber became blocked then the fan would find it easier to reach the target pressure and hence would actually slow down, again exacerbating the problem.

Here the transducer/transmitter is measuring resistances which are a function of the full combined flow. This position is no good if the objective is to respond to individual line closures upstream of the fan. In the original case where the objective is to reduce flow in response to a closure of one of the upstream lines, the pressure at the point of measurement would decrease. The fan would then speed up to correct the pressure and would consequently extract more flow from the remaining points.

If the pressure transducer/transmitter is moved downstream of the adsorber then the pressure response would favourably control the fan in response to the adsorber being blocked but again, would not allow control in response to the upstream individual legs.

A means of addressing both scenarios would be to have variable pressure set-points in the controller. This means that there would be 5 pressure set points assuming all 5 of the emission points has the same extract or a larger multitude of set points if the extracts are of differing values. The required static pressure at the point of measurement would have to be determined for each combination in order to capture all the relevant system pressures. The system would also have to receive signals to indicate that one or more of the extracts has been closed (e.g. limit switches).

Obviously, a simpler solution is not to vary the flow at all but to maintain extract at all points even if it is not required. This is ok if the energy consumption is low for the full extract requirement but would be costly for higher flows.

Flow Control

Pressure control can be effective, but pressure is not necessarily proof of flow and direct flow or velocity measurements offer a more robust approach. The placement of a flow transducer/transmitter is not as critical as the pressure scenario however the device must be exposed to a relatively stable velocity profile (ideally 3 diameters downstream of a disturbance or at least 2 diameters upstream of a disturbance). It is not possible to capture the variance in flow with a single set point as with pressure and so the multiple set point approach would be required for a single transducer.

If one or more of the extract lines is critical (e.g. Zoned area required extract rate) then individual flow switches should be used to raise alarms in those extracts. This is because often the reduction or even loss of an individual flow may be less than the alarm setting for the combined flow. As an example, a combined flow may be 25,000 m3/hr and a sensible alarm setting may be 20,000 m3/hr. In this case any extract less than 5000 m3/hr would not trigger the alarm and hence its loss would go unnoticed. Note it may not be possible with the accuracy of flow control and monitoring to streamline the alarm to be smaller gap. In our example if we were to set the alarm at say 24,000 m3/hr then we would more than likely experience numerous nuisance alarms as natural fluctuations would often exceed this bandwidth.

HAZOP

The issues faced with extraction and the nuances of pressure/flow control lead us to conclude that any extraction with any level of sophistication or safety concern needs to be addressed by a HAZOP or other rigorous, structured safety review.

The position of instruments the, way extracts are connected, the consequences of reduced extract or mixed extract are all important considerations and sometimes the optimum solution is counter-intuitive.

We would always recommend a HAZOP study for extract systems, even if there is no accompanying abatement system.

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Waste Handling Odour Control in Transportable Containers
April 14, 2017, 3:03 pm
Filed under: Uncategorized

When dealing with high flow odour control systems ( e.g. waste Transfer Stations or Waste reception facilities) then we can design a variety of different orientated systems within transportable freight containers to save cost and minimise footprint.

Our designs can handle up to 67,000 m3/hr per system and typically include for the dust filtration, extraction duct as well as the extraction fan and industrial carbon adsorber itself.

In this example the System air flow requirement is 66,000 m3/hr nominal and the project included the entire system with exhaust stack. The abatement itself included 2 stage filtration  prior to the adsorber to prevent the carbon from being blinded by dust. In this case the adsorber has 2 internal beds with access for extraction and replenishment from the top. An internal baffle arrangement ensure full carbon utility and even flow through the beds.  01534644cfc1d8cc6acce003aeabcc1f7bfa83d404

The above project was undertaken through Filtrex Limited and with manufacture and installation contributions from Metalcraft Ltd.

What other Technologies are viable for the Waste Industry?

 

There are a range of different waste handling processes and waste types. They range from Food waste handling ( often with Anaerobic Digestion plants) to municipal and green waste handling. Within the waste types there are transfer stations, autoclaves, Dirty and clean side MRFs, Hammer mills, incineration and numerous other combinations of processes. Each of the different types of process require different approaches.

We can design systems based around adsorbers, biofilters, Regenerative or recuperative oxidisers and various types of chemical scrubbers. Sometimes a process may require more than one technology depending on the stack emission consent.

The extraction system is vital in terms of ensuring capture and containment of odours with the minimum amount of required air. The optimisation of extract air is a broad and often political aspect of the design basis. We tend to use CIBSE guides and then Chemical Engineering calculations to interrogate the process and determine the best extract philosophy. Only then can you hope to determine the optimum abatement technology.

For more details contact Andrew Piearcey : Andrew@askpiearcey.co.uk  or 07877 456718



HAZOP Service Launched
September 16, 2014, 1:09 pm
Filed under: Uncategorized | Tags: , , , , , , ,

ASK Piearcey Ltd have developed their own in-house, database driven, HAZOP process. We can now facilitate entire HAZOP events or attend as a contributor or scribe. So What is a HAZOP??

According to the  Health and Safety at Work Act 1974 (HASAW) and for Major Hazardous processes COMAH (1999) regulations, it is the responsibility of the OPERATOR or end user to identify risks associated with a plant.

 

There numerous regulations and protocols to refer to ( example see HSE website: http://www.hse.gov.uk/comah/sram/index.htm ) but in our view, the End-User or operator of the plant must facilitate and attend a HAZOP for the plant as a minimum and must fully discharge their responsibilities under HASAW and where appropriate, COMAH.

 

A HAZOP is a study and NOT a document. Any HAZOP documentation is a product of a HAZOP study.

 

The HAZOP study should include but not be limited to a team of individuals with responsibility within the project to be studied as follows:

 

  • An electrical engineer
  • A control/software engineer
  • A mechanical engineer
  • A process engineer
  • An operator (as in person who will be physically operating that plant)
  • Project manager(s)
  • Engineers relating to specific plant items (e.g. a representative from a reactor manufacturer
  • Persons in positions of responsibility for the End User
  • Persons in positions of responsibility for the Principal contractor
  • Persons in positions of responsibility for the Planning Supervisor
  • An independent chairperson
  • An independent scribe

 

The HAZOP centres around the “frozen” process and instrumentation diagram (P&ID). It is a rigorous, painstaking and methodical study of each and every line within the P &ID diagram (referred to as “nodes”). Each node has a list of guide words applied to it and each guide word has a list of deviations applied to it in turn.

 

Example:

 

Guide word is “Flow”

 

Deviations maybe :

“reverse”

“None”

“misdirected”

“Less than”

“More than”

Each application of a deviation will generate scenarios, discussion, questions about the risk and consequence of the deviation and at each stage the plant is questioned for its robustness and suitability to prevent, minimise or handle such deviations. This in turn generates actions for individuals present to rectify any perceived flaw or weakness in the design.

We have never encountered a HAZOP which has not changed the design in some manner and hence it is extremely important that a HAZOP is carried out.

HAZOP image 1 HAZOP image 2

A HAZOP may take some weeks to organise, some days to complete and some further weeks to sign-off any actions. The Actions may result in the change of the P&ID, operational instructions, drawings and design. A design cannot therefore be fit for purpose unless it has undergone a formally recorded and signed-off HAZOP.

With our  in-house designed HAZOP program, we  can carry out formal HAZOP studies. We have successfully managed and chaired HAZOPs using our database centred system. There are also companies and individuals who specialise in HAZOPs and indeed other Structured What-If Techniques (SWIFT) and they  also have fully contained database driven HAZOPS that include the procedures for issuing and signing off actions. We advise that end-users formally engage  ourselves or other specialists to facilitate HAZOPs if they do not have the capability in-house.

The HSE also advise a HAZAN (for COMAH) is undertaken as part of the hazard identification process. This is a study of the severity of a hazard in the event the risk of the hazard is realised.  In our experience, HAZANs are often not a formal, separate study but are split into actions for groups of individuals. It is advisable to fully explore the consequences of a hazard actually occurring as it may well significantly affect the methods of mitigation put in place. An example would be the potential for cross filling of sulphuric acid into a chemical tank containing sodium hypochlorite. In this event chlorine is formed and this is a highly toxic gas. At a HAZOP measures would be suggested to minimise the risk of the cross filling occurring. The HAZAN looks at what would happen to chlorine if it were formed and what consequences this would have. If the implications included  multiple fatalities then the HAZOP actions may well be made even more robust and the “budget” for mitigation increased accordingly.

It should be noted that in our experience as attendees, the “OP” part of HAZOPs is often left relatively unexplored (particularly when operators are not involved in the study) and another study known as an ALM (Access Lifting and Maintenance) can be useful in identifying and eliminating operational issues. Of course Access, Lifting and Maintenance would  simply be added as guide words to the HAZOP  if we were managing  the event and they would probably have no deviations.

For complicated projects, especially those involving several stages of treatment and with numerous interfaces with other equipment, it is advisable to implement a HAZCOM. This is a “big picture” SWIFT technique which looks at a whole process rather than individual lines on a P&ID. The purpose of a HAZCOM is to identify hazards that may arise as a result of commissioning activities. The HAZCOM needs to involve representatives from any party involved in concurrent commissioning activities. It becomes particularly relevant when there is dependency between various parties during the commissioning phase.

ASK Piearcey Ltd have attended HAZCOM events and found them to be useful in raising awareness of concurrent commissioning activities and their associated hazards.