Process Safety Tips

8/18/2019 Process Safety Tips

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Process Safety Tips

D.M. Butala

We are measured not by what we are,but by the perception of what we seem to be;not by what we say, but how we are heard,

and not by what we do, but how we appear to do it.-- (Author Unknown)

Process safety is an important aspect at the design stage in plant operation and modifications that needs to

be addressed at the design stage. As designers, we must design the procedures, the equipment and controldevices to make unforeseen equipment failures and operator mistakes as rare as possible.

Modifications are carried out to increase production, to compensate for unavailable equipment, to increasestorage capacity, to improve yields, to reduce costs, to enhance safety and to reduce pollution potentials.To achieve these goals, changes in piping or equipment, introduction of new operating procedures, newoperating conditions, changes in materials of construction as well as changes of process chemical in

feedstock, catalyst, fuels, or their methods of delivery, etc., have to be considered.

In industry, accidents / disasters do take place. We learn lessons from such incidents. There are manymyths prevailing in chemical process industry (CPI), which are the sources of disasters. Misinterpretationof the causes of these disasters can result in bad safety procedures that lead to even more disasters. Usinglimited resources and paying attention to prevent a recurrence of the last accident only prevent people fromseeing what really needs to be fixed.

Have we failed to learn from the past incidents and mistakes? If yes, we will do what we have been doingand continue to face the results. So, let us give a fresh look at the aspect of process safety as there should be no compromise and complacency in matters of safety.

Here are some tips to differentiate myths from realities for improvement in process safety.

1. Earthing:

People believe that as pipings are connected by flange joints, there is no need for earthing around these

joints. People also believe that vessels and storage tanks used for hydrocarbon products, rest on their

support legs with foundation bolts. Hence, there is no need to earth them. This is a myth. There should be

jump-over connections with cable on both the adjacent flanges of the piping for hydrocarbons and vessels

should be adequately earthed to avoid accumulation of static electricity charges.

Many operating personnel believe that since road tankers rest on rubber tyres, there is no requirement for

earthing in tankers during loading / unloading operations. This is a myth. Accidents have occurred due to

static electricity generation and lack of earthing. While loading or unloading hydrocarbons, road tankers

should be properly earthed with crocodile clamps.

The author is a senior industry professional having sound experience in Project Engineering, Management, Process Design,

Process Safety Design, Safety Management, Plant Operation & Maintenance, Plant Debottlenecking in the field of Fertilizers,

Plastics, petrochemicals, Refinery, Product Development and R&D.


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2. Insulation:

Insulation is used for hot and cold services. However, the need for integrity of insulation is overlooked.

External Corrosion

For running plants, once insulation is carried out at the project stage, by and large it remains unattended.

Over a period, the protective covering of insulation - aluminum or cementing - as the case may be becomes

loose permitting ingress of rain water or condensation of atmospheric moisture. In course of time, this

causes external corrosion of pipe and equipment reducing thickness and making the system vulnerable to

leakage or rupture of pipes. It is worth checking the insulation integrity during every annual shutdown.

One should also check the external surfaces of the materials susceptible to corrosion once in 3 ~ 4 years as

part of the plant preventive maintenance programme.

Inadequate thickness and improper selection of insulation:

Inadequate thickness of insulation and wrong density of the insulating media lead to energy losses. Many

companies, particularly the small and medium enterprise (8ME) sector, use insulating ropes to save cost.

However, in the process, as there is a fine gap between successive turns, insulation loses its effectiveness.

Use of proper insulating material with right density:

Insulating mats of different densities are prepared using linear resins for bonding the insulating fibers, e.g.,

mineral wool fibers. Often insulating contractors use low-density mats for temperature above 200 deg. C.

At higher temperature, bonding resin gets evaporated, making insulation porous and permitting air passage,

thus, resulting in loss of heat.

Lagging Fire:

When mineral wool insulation is used for heat transfer oils (thermo-fluids), leakage from flange joints

causes soaking of insulation by heat transfer oils (thermo-fluids). This leads to thermal degradation leading

to lowering of auto ignition temperature and polymerization of entrapped oil. This generates lagging fire

which can be a potential catastrophe. Hence, it is better to inspect insulation after every equipment

maintenance work and replace faulty tools at the next available opportunity.

Solids entrapped in insulation impacts flexibility of piping:

Processes for Phthaleic Anhydride, Melamine, etc., have cycles of operation where the product at high

temperature is in sublimation state. Leakage through flanges or expansion bellow permits ingress of

sublimed products into insulation which solidifies in the insulating material, making it heavy and

ineffective. This is also the case where molten salt is used as heating media / high temperature heat sink. It

is worth replacing such damaged insulation at the earliest shutdown opportunity.

3. Modifications:

Due to urgency, downtime, production loss, management pressure, etc., and at times, due to complacency,

modifications are carried out and short cuts are adopted without safety review and proper design checks.

Further, there are unforeseen additional shut-down jobs as well as budget constraints.


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Practices adopted are:

Temporary connection during shutdown

Poor draining of process lines or lines having dead pockets filled with process fluids, such as, acids

or hydrocarbons.

Improper isolation of connected lines and vessels (Valves not holding)

Poor access / lack of proper scaffolding due to poor workmanship

Improper selection of gaskets

Bypassing of trips

Poor electrical isolation

Temporary modifications treated as permanent modifications, etc.

Such practices and short cuts should be avoided. Prior to modification, a thorough design check and safetyreviews must be carried out.

4. Safety Relief System:

• For hazardous services, safety relief valve vents should at least be 2 m above the operating

platforms.

•

There should not be isolation valve upstream to safety valves. In case it requires to be installed, it

should be 'car seal' opened.

• For choking service (e.g., polymerization reactors), rupture disc should be provided upstream with

safety relief valves.

• During planned maintenance, all safety valves should be properly labelled to avoid remounting

them at wrong locations. Also, lapping, resetting and set pressure testing of safety valves should be

carried out during overhauling of plants.

•

If rupture disc (RD) is selected for pressure relieving, reverse buckling-type RD should be

selected. For polymerization service, RD should be reverse buckling with knife edge cage.

• If it is an old plant or if it is not possible to route vents of safety valves for toxic or hydrocarbon

service to closed blowdown header, vents should be raised to 2 m above the operating platform.

Moreover, at outlet of vent, snuff steam provision should be made.

• For hydrocarbon or acid service, like such as, HCl (Hydrochloric acid), concentrated H2SO4

(Sulfuric Acid), Oleum, etc., the inlet lines to storage tanks should be with 'block &bleeder' (two

valves with a drain valve in between).

• There should be independent blow-down headers for high pressure and low pressure safety relief

valves. Or else, there will be back pressure on safety valves in the low pressure system.


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5. Controls:

• Seat and plug of control valve should be made of wear resistant materials. Stelliting of seat and

plug will prevent problems of improper functioning of the control valves.

•

By-pass provision for control valves should be made for full capacity unless licensor permits based

on his experience.

• Signal against tripping should be generated from an independent transmitter and not from the

transmitter used in the control loop.

• If runaway reaction is to be arrested by water quenching in the reaction media, quench water

pressure should be higher than the reaction operating pressure. Gravity quenching should be

avoided.

•

Pressure controls should be used in distillation columns handling hydrocarbon or toxic fluids andoperating under vacuum/ using inert gas (Nitrogen).

6. Storage Tanks:

Vents:

• Vent size should at least be the same as that of the largest inlet or outlet nozzle or it should be one

size higher to take care of any collapse due to pulling of vacuum during draining.

•

The opening of vent should be protected with wire gauge to prevent bird entry.

• Vents should not be blocked while draining or else pulling of vacuum will lead to collapse of the

storage vessel.

• Vents on concentrated sulfuric acid (H2SO4) storage tanks should be provided with moisture trap

or else the roof will get corroded.

• Vents on demineralised water (DM) storage tanks should particularly be provided with caustic trap

to avoid ingress of carbon dioxide (CO2) from the atmosphere.

• Vents of all hazardous / toxic service shall be routed through scrubber at safe location or through

flares as the case may be.

Other Nozzles and Mounting on the tanks:

• For hydrocarbon service, the vapour seal box mounted on foam inlet nozzle of the tank should be

provided with glass which breaks at 3.5 kg/cm2 pressure of foam. This is not to be treated as

overflow nozzle.

• For hazardous / HCs (Hydrocarbons) services, no overflow nozzle should be provided.

• Lightening arrestors should be provided with earthing.


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• For large capacity sulfuric acid (H2S04) or oleum storage tanks, the bottom outlet nozzle should

be provided with plug valve at nozzle inlet inside the tank and it should be capable of operating

from the tank roof.

• Tanks fitted with internal heating arrangements should be filled maximum up to 80 % capacity and

provided with temperature control to prevent boil over. (e.g., FO (Fuel oil) storage tanks.

• Flexible hose for rain water drain connection with floating roof tanks should be periodically

inspected during the overhauling of tanks.

• If bigger capacity fixed-roof storage tanks are used for low boiling point HCs, the same should be

provided with water spray from top and spiral staircase at the outer side of the tank shell wall to

minimize breathing losses in summer.

7. Cooling Towers

• The return cooling water (CW) header for large cooling towers should be located at the ground

level and risers should be taken to the deck of cooling towers to avoid damage and collapse of the

structure because of water hammering and weakening of woods in the structure over a period.

• There should at least be a clear distance of 15 m between the cooling tower and the nearby plant

structure for unrestricted flow of air to cooling tower.

• If foaming is noticed from the cooling tower, there should be checks for leakage in heat

exchangers installed for the HCs service.

• Circulating cooling water pH should be controlled with H2SO4 and not with nitric acid (HNO3) or

hydrochloric acid (HCl).

• Addition of Chlorine to cooling tower should ensure that residual Chlorine in cooling water does

not exceed 0.5 ppm.

8. Vessels, Piping, Plant Start-ups:

• Forty per cent of all major process plant losses are due to failures in piping.

Knockout pots (Separators):

• Knockout pots for compressors should be provided with adequate disengagement height to avoid

liquid droplets carryover to compressor suction.

Dust handling system:

• Tall vertical silos / bunkers used for storage of solids should be adequately earthed to avoid dust

explosion due to static electricity.

•

To break arching while withdrawing solids from the bottom of silo, use of air blasters is

recommended.


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Process drain mains:

• The intermediate collection pits, connected with the main process drains in plants, should be

designed with underflow-overflow baffles as guard against propagation of fire in process drains.

Reformers:

• Reformer tubes are fitted with tension adjustment. During operations, because of heat, the tension

in each tube is to be readjusted to avoid failure because of thermal stresses generated due to

thermal expansion.

Expansion bellow joints in heat exchangers and piping:

• Guards fixed on expansion bellow to protect against damage during transportation / erection

should be removed during commissioning to avail of the advantage of expansion bellow.

Startups/ Normal operation /Maintenance:

• For startups or during pre-commissioning activities, vessels and pipings are steamed out. As steam

condenses, vessels may collapse due to vacuum generation as these are not designed to operate

under vacuum because normal procedures do not call for such operation. Hence, care should be

taken to prevent the possibility of vacuum generation.

• Chlorine from chlorine cylinders is bubbled directly in to the reactors. This can lead to reversal of

reactants to Chlorine cylinder / tonner and create catastrophe. Therefore, an inverted 'U loop' of

minimum 10 m-height should always be provided.

• If the column is not protected against vacuum, during draining of water after it is flooded with

water for cleaning operation, the column may collapse due to pulling of vacuum.

• Many people do not realize that there is massive increase in volume caused due to vaporization of

water into steam. This leads to explosion of vessels as they are not designed for such conditions.

Hence, operators should keep safety aspects in mind when running this procedure.

Process Safety Tips

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