Transfer of dangerous liquids
- Hazards associated with handling dangerous chemicals
- Limitations of shaft-sealed pumps
- Leak-free pumping solutions
Chemicals are considered dangerous if they are toxic, highly corrosive or volatile. Chemicals can be dangerous for several reasons; they can affect a person’s health, cause litigation issues in case of serious harm and they can pollute the environment and our food chain, such as with pesticides.
For industrial premises, these dangers place a great responsibility on the staff, procedures and plant equipment to be safe and reliable. This article is concerned with the pumping considerations for the safe handling various dangerous chemicals.
The danger of chemicals
Many chemicals and solvents are used in the production of all kinds of consumer goods and industrial processes. These chemicals can be used as part of the production process such as in chemical processing, the pharmaceutical industry or the treatment of effluent. Chemicals may also be used to clean, treat or finish products, for example in the pickling of metals or cleaning of equipment which is used in hygienic food processes (E.g. Sodium Hydroxide in a CIP process). Chemicals may also be constituent parts of a solution such as Toluene or Methylbenzene, which are commonly used as solvents in adhesive compounds such as superglue. When inhaled, these solvents can affect the central nervous system, for example amnesia and is a known trigger for cancer.
Sulphuric acid is the most commonly used acid in industry. It is used in the production of fertilizers and as an electrolyte in car batteries. Sulphuric acid in contact with skin will cause severe burns; therefore, it is important to set up the production process in such a way that the risk of leakage when pumping chemicals is eliminated.
The background presence of chemical is also significant. Hydrochloric acid for example can attack the eyes, tooth enamel and the respiratory system, causing long-term health implications for employees. Where the chemical is highly poisonous, such as hydrofluoric acid or volatile such as acetone, it is critical to minimise the background vapour presence for the safety of the plant as a whole.
Often the chemicals arrive at the plant in drums or containers, from which they are transferred into the production area to reactors, storage tanks or production lines. During the production process they become diluted, mixed with other chemicals or solids in suspension. As the chemical becomes mixed with different kind of materials, it may need to be passed through a filtration system to recover the original chemical. As these chemicals pose a threat to the local environment, they require handling in such a way that a risk of leakage or incident is mitigated or removed completely.
Leak-free operation and containment of the media
The sealing system is a critical aspect in the safe operation of a pump.
“A Pump is only as good as its seal” is a known saying in the world of industrial pumps. The pumping principle often places limitations on the type of sealing which can be used. This is explained in detail by comparing shaft-sealed pumps to sealless pumps.
Rotating shaft-sealed pumps
Most shaft-sealed pumps E.g. rotary lobe pumps, centrifugal pumps, gear pumps and vane pumps require a sealing mechanism between the rotating shaft and the liquid being pumped.
The simplest method is to use gland packing. A disadvantage when using packing rings is the generation of heat, which can cause problems in explosive dangerous areas. In addition, a packing can only work by using the pumped liquid as lubrication and coolant for the packing rings. A packing gland needs continuous maintenance as due to abrasive wear, the packing rings needs to be tightened, however the inherent risk of leakage is always present. This problem can be exacerbated by fluids which crystallize on contact with air, such as varnishes increasing the abrasive wear on the packing.
To lessen the risk of evaporation of the liquid, the gland packing can be equipped with a lantern ring and a quench (see fig 2). However, the pumped liquid may still penetrate through the packing rings, which are before the lantern ring and will be diluted in the quench liquid. In the event of the barrier fluid level and/or pressure dropping, an alarm will trigger, indicating a failure and for the pump to be serviced. For the end-user this type of pump installation is costly in terms of the replacement seals, barrier fluid, flushing and fitting time as well as the cost of downtime. Should the system not be alarmed, a catastrophic leak is inevitable.
A mechanical seal provides a more secure solution and is a much more common feature in process pumps than packed glands. The mechanical seal has the added benefit of sprung loaded tension creating a continuous sealing face with the process liquid acting as the lubricant and coolant between the seal faces. As the mechanical seal is rotating against a stationary face the seal will be subject to abrasive wear. The result of using a single mechanical seal can see the fluid ‘weeping’ and evaporating, causing a background presence of vaporised chemical or at worse, a ‘release to atmosphere’ of the liquid.
Where a double mechanical sealing system is in place, the primary seal will either require inspection and eventually replacing or a failure of the seal from abrasive wear will cause a drop in pressure or fluid level in the quench, creating an operator alarm. Should the pump not be alarmed or not attended to, the secondary seal may also fail, causing a release to atmosphere.
It is rare to see a release to atmosphere incident with a double mechanical sealed pump; however the use of this system does not come without a financial cost. The pressurized quench system requires a thermo-siphon system and/or an extra pump to maintain the pressure in addition to the cost of 4 seal faces and the labour cost of installation, inspection and servicing.
B Sealless pumps
Although termed ‘sealless’, these pumps do in fact have seals, usually o-rings creating a static seal between the flange faces of casing sections. The critical aspect is where the seals are located as the term sealless refers to the pump having no shaft seal in contact with moving parts, so they are not subject to abrasive wear from a rotating mechanism. The fluid instead is handled in a closed pump chamber, such as in magnetic driven pumps, peristaltic pumps and air operated double diaphragm pumps.
Providing the materials that come into contact with the process fluid are chemically compatible, the pump will provide total containment.
The mag drive solution
The wetted parts of mag-drive pumps are commonly made out of stainless steel, plastic or metal lined with a polymer, such as ETFE. A lining can be an excellent solution as it provides the chemical resistance against corrosive fluids as well as the strength and durability of a metal casing. Many mag drive pumps operate in processes for years, even decades without any breakdown issue. This is due to the inherent robustness of the working principle. Under normal working conditions the only wearable part is the bearing assembly, which typically should be inspected every 2 years with an expected service life of 5 years if the liquid is clean and non-abrasive. Many chemical manufacturers with ATEX rated zones, refineries and remote locations standardise on mag drive units because of the performance and reliability of these units.
Where a fluid is more viscous than 200-250cPs or contains solids, mag drive pumps would be unsuitable as the fluid will not enter the flow path easily enough as well as excessive loads being placed on the bearing and magnet assemblies. Hard solids such as grit or sand will certainly wear the pump very quickly and even cause catastrophic failure if the solid is larger. At this stage, a positive displacement principle would be more appropriate. An example of this may be in the manufacture of paints and lacquer where poisonous, viscous resins containing cyanide compounds are in use. (For a more in-depth look at mag drive click here...)
The positive displacement alternative
The hose itself can be produced from various rubber compounds such as natural rubber, nitrile, EPDM and Hypalon. With every revolution of the shoe and rotor, the hose undergoes compression and release, relying on the elastic and durable properties of the hose. This eliminates thermo-plastics and metallic materials due to their brittle and rigid nature.
Peristaltic pumps are therefore limited in pumping solvents and other chemicals which are not compatible with rubber.
Over the lifetime of the hose, the elasticity of the rubber hose will degrade, reducing suction capability and the flow rate. Known as ‘spellation’, as the hose is prone to wear, pieces of rubber can begin to break from the hose and will be transferred into the liquid.
All hoses are subject to wear and will eventually fail resulting in the rupture of the hose, causing the process liquid to enter the casing, commonly made from cast iron or aluminium. Peristaltic pumps can be fitted with burst hose protection which detects a build up in pressure. The build-up in pressure triggers a sensor for the pump to be attended to. Should the pump be left unattended the internal workings of the pump such as the casing itself, bearing and drive assembly and even the gear box can be damaged beyond repair as these are only protected by a simple lip seal which is easily breach in a build-up of pressure and/or chemical attack from the process liquid.
Air operated double diaphragm pumps
Standard air-operated double diaphragm (AODD) pumps are made of metal or injection moulded plastics such as Polypropylene or PVDF. These materials can handle most common chemicals and solvents; however for some chemical compounds it is difficult to select the correct pump material. If higher temperatures are needed, the moulded plastic pumps are limited to +90°C. The diaphragms can be made out of different materials (rubbers or thermoplastics) but for chemicals and solvents mostly PTFE diaphragms are used. The diaphragms separate the pumped liquid from the air chamber.
Different sections of the pump may have restrictions due to the properties of materials such as rigidity, temperature limitations, chemical resistance or the expense. An example may be a polypropylene air section and the pump casing made from PVDF.
In the case of a diaphragm rupture, the liquid will pass the diaphragm and enter the air section. If this is not made from a material which is 100% chemically resistant, the air section will be attacked by the medium. A leak detection sensor placed in the air section can help detect a rupture of the diaphragms but it is far from ideal as the sensors are placed in the air side of the pump. The detection of a rupture will only happen when the pumped chemicals already have escaped to the compressed air and can be moved out of the pump to the atmosphere before the sensors will detect the breach.
Specialist chemical AODD pumps
Designed and manufactured specifically for the handling of corrosive chemicals in the most hazardous environments, machined AODD pumps made from pure virgin PTFE are available for the most critical applications.
Virgin PTFE is chemically inert, so it will not react with any medium. Unfortunately it is impossible to use an injection moulding process for virgin PTFE; therefore pumps made out of this material require machining from a solid block of material. End users benefit as by using this pump they can handle any chemical or solvent without risk of corrosion causing a release of hazardous fluids.
Where the area of operation is rated as hazardous due to risk of explosion (ATEX), a small amount of carbon can be added into the PTFE. The carbon molecules are completely surrounded by PTFE molecules, so the chemical properties will remain the same, however the carbon will make the PTFE electrically conductive to meet the requirements of the European ATEX 94/9/EG directives.
Extra safety measures
As with all AODD pumps, the same issue of a ruptured diaphragm occurs, with the pumped liquid entering the air-side of the pump. Because this type of pump is machined and not moulded, it is technically easy and cost-efficient to add on accessories such as barrier chambers and drain channels to add an extra line of safety for the workforce and surrounding environment.
A barrier chamber is a solution to the inherent problem of a ruptured diaphragm, which would allow a hazardous fluid in to the air chamber and eventually out through the exhaust before a sensor can shutdown the flow of the liquid.
The barrier system works by adding an additional diaphragm between the air and process fluid to act as a barrier if the primary diaphragm ruptures. This creates an additional chamber, which is filled with a neutral fluid such as demineralised water and two sensors detecting the presence of the neutral fluid and another for the state of electrical conductivity of the fluid. In the event of a diaphragm failure the hazardous fluid alters the state of the neutral fluid, resulting in a change of conductivity, which is relayed by the sensor to alert the operator.
Due to the working principle of an AODD, one of the pump chambers will always be filled with liquid, even when the pump is stopped. The suction valve balls prevent the flow of liquid from the pump.
When a pump system needs to switch from one medium to another, it is important no liquid is remained in the pump chamber to avoid cross-contamination and chemical reactions in the pump when the new liquid enters the pump chamber.
When maintenance is performed, it is also important the pumps do not contain liquid in the pump chambers.
As the walls of a solid machined AODD pump are relative thick, it is possible to drill channels in the side housing without affecting the structural integrity of the housing. These drain channels make it possible to drain the pump before switching to another medium or before starting maintenance work.
The critical points when choosing a pump to handle hazardous chemicals and solvents are:
- The correct pumping principal for the fluid characteristics, system and the frequency of operation.
- Leak-free operation and/or containment of the media in case of failure
- Chemical compatibility for all wetted parts (and parts in contact with the medium in case of a breach of the primary containment method).
- The pump is easy to operate or is “foolproof”
- Installation and maintenance can be done in a safe and cost-effective way
- It is important to make an in depth risk analysis of your work environment and pumping systems to be sure that the transfer of dangerous liquids is done in a safe and reliable way.
About the author:
Wim Rochtus is product and development manager for Verderair, part of the Verder Liquids division.
The manufacturing activities of the Liquids Division of the Verder Group focus on the production of innovative positive displacement pumps.
Wim Rochtus can be contacted by mail: firstname.lastname@example.org.