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OISD-109 Amended edition FOR RESTRICTED CIRCULATION
PROCESS DESIGN AND OPERATING PHILOSOPHIES ON
BLOWDOWN AND SEWER SYSTEM
OISD-STANDARD - 109 First Edition, November 1988 Amended edition, August, 1999
Oil Industry Safety Directorate Government of India
Ministry of Petroleum and Natural Gas
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OISD-STANDARD-109 First Edition November 1988 Amended edition, August 1999 FOR RESTRICTED CIRCULATION
PROCESS DESIGN AND OPERATING PHILOSOPHIES ON
BLOWDOWN AND SEWER SYSTEM
Prepared by : COMMITTEE ON
PROCESS DESIGN AND OPERATING PHILOSOPHIES
OIL INDUSTRY SAFETY DIRECTORATE 2nd Floor, Kailash,
26, Kasturba Gandhi Marg,
IV
NEW DELHI 110 001
NOTE OISD publications are prepared for use in the oil and gas industry under the administrative control of Ministry of Petroleum and Natural Gas and shall not be reproduced or copied and loaned or exhibited to others without written consent from OISD. Though every effort has been made to ensure the accuracy and reliability of the data contained in these documents, OISD hereby expressly disclaims any liability or responsibility for loss or damage resulting from their use. These documents are intended to supplement rather than replace the prevailing statutory requirements.
Note 1 in superscript indicates the modification/changes/addition based on the amendments approved in the 17th Safety Council meeting held in July, 1999.
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FOREWORD The oil industry in India is nearly 100 years old. As such a variety of practices have been in vogue because of collaboration/association with different foreign companies and governments. Standardisation in design philosophies, operating and maintenance practices at national level was hardly in existence. This, coupled with feed back from some serious accidents that occurred in the recent past in India and abroad, emphasised the need for the industry to review the existing state of art in designing, operating and maintaining oil and gas installations. With this in view, the Ministry of Petroleum and Natural Gas in 1986 constituted a Safety Council assisted by the Oil Industry Safety Directorate (OISD), staffed from within the industry in formulating and implementing a series of self-regulatory measures aimed at removing obsolescence, standardising and upgrading the existing standards to ensure safer operations. Accordingly OISD constituted a number of functional committees comprising of experts nominated from the industry to draw up standards and guidelines on various subjects. The present document, on Blowdown and Sewer System was prepared, by the Functional Committee on Process Design and Operating Philosophies. This document is based on the accumulated knowledge and experience of industry members and the various national and international codes and practices. It is hoped that the provision of this document, if implemented objectively may go a long way to improve the safety and reduce accidents in the oil and gas industry. Suggestions are invited from the users for further improvement in the standard after it is put into practice. Suggestions for amendments to this document should be addressed to :
The Co-ordinator, Committee on Process Design and Operating Philosophies
Oil Industry Safety Directorate 2nd Floor, Kailash,
26, Kasturba Gandhi Marg, NEW DELHI 110 001.
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COMMITTEE ON
PROCESS DESIGN AND OPERATING PHILOSOPHIES
List of Members S.No. Name Designation & Position in Organisation Committee
1. Shri W.D. Lande Dy. Gen. Mgr. Member HPCL Leader 2. Shri G. Raghunathan Ch. Manager, Co-opted HPCL Member 3. Shri N. Lal Dy. Gen. Manager, Member ONGC 4. Shri N.N. Gogoi Dy. Gen. Mgr., Member OIL 5. Shri S.V. Puthli Sr. Manager, Member HPCL 6. Shri M.A. Sreekumar Sr. Manager, Member CRL 8. Shri A. Vardarajan Sr. Manager, Member MRL 9. Shri B.K. Trehan Addl. Director, Member, OISD Co-ordinator In addition to the above, several other experts from industry contributed in the preparation, review and finalisation of this document.
PROCESS DESIGN AND OPERATING PHILOSOPHIES ON
BLOWDOWN AND SEWER SYSTEM
CONTENTS
SECTION 1.0 INTRODUCTION 2.0 SCOPE 3.0 CLASSIFICATION OF BLOWDOWN STREAMS 3.1 Classification Based on Composition 3.1.1 Aqueous Blowdown 3.1.2 Hydrocarbon Blowdown 3.1.3 Chemical Blowdown 3.2 Classification based on Temperature 3.2.1 Hot Blowdown 3.2.2 Cryogenic Blowdown 3.3 Classification Based on Volatility & Viscosity 3.3.1 Volatile 3.32 Non-Volatile 3.3.3 Viscous/Congealing 4.0 SAFE HANDLING 4.1 Aqueous Blowdown 4.1.1 Aqueous Blowdown - Continuous in Nature 4.1.2 Aqueous Blowdown Likely to Contain Volatile Hydrocarbon 4.1.3 Boiler Blowdown - Continuous 4.1.4 Water Draw-off from Crude Oil Tanks 4.1.5 Other Aqueous Blowdown - Intermittent in Nature 4.2 Hydrocarbon Blowdown 4.2.1 Draining/Venting from LPG/Cryogenic Product Pumps 4.2.2 Draining/Venting from LPG/Cryogenic Product Pumps 4.2.3 Draining of Equipment for Shutdown 4.2.4 Sample Points 4.2.5 Gas-Condensate Blowdown 4.3 Emergency Blowdown 4.3.1 Blowdown from LPG and Other Cryogenic Vessels 4.3.2 Furnace Blowdown 4.4 Chemical Blowdown
5.0 DISPOSAL OF BLOWDOWN 5.1 Closed Pressured System 5.2 Sewer Drain System 5.3 Type of Sewers 5.3.1 Type of Sewers 5.3.2 Storm Water Sewers 5.3.3 Chemical Sewers 5.3.4 Sanitary Water Sewers 5.4 Basic Design Guidelines for Oily-Water Sewers 5.4.1 Layout of Oily Sewers 5.4.2 Capacity 5.4.3 Material of construction 5.5 Special Sewers 5.5.1 Asphalt Sewers 5.5.2 Waxy Drains 5.5.3 Drains at Remote Locations 6.0 REFERENCES FIGURES 1a, b Aqueous Blowdown - Continuous in Nature 2. Aqueous Blowdown - Likely to Contain Volatile Hydrocarbons 3. Continuos Boiler Blowdwon 4. Water Draw off from Crude Oil Tanks 5. Draining of LPG Vessels 6. Draining/Venting of Hot Pumps 7. Sample Points 8. Sample Points 9. Gas-Condensate blowdown 10. Blowdown Drum 11. Blowdown Drum 12. Quench Drum 13. Typical Single Compartment Sealed Manhole 14. Typical Double compartment Sealed Manhole
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PROCESS DESIGN AND OPERATING PHILOSOPHIES ON
BLOWDOWN AND SEWER SYSTEM 1.0 INTRODUCTION Blowdown as defined in this document is a liquid stream containing water, oil, chemicals or a combination of two or more of these which are required to be drained from various process equipment in the hydrocarbon industry under different operating situations like start up, shutdown, normal operation or emergencies. Since majority of these streams are usually waste products (however in some situations these may be recoverable), their handling and disposal often gets a low profile in the plant design and operation. However, the importance of handling these blowdown streams in enhancing the safety of the plant cannot be underestimated. The purpose of this document is to recognise the various blowdown stream s commonly encountered in the oil and gas industry and give guidelines for the safe handling and disposal of some typical streams.
2.0 SCOPE This document covers the guidelines on design and operating philosophies for safe handling and disposal of liquid blowdown and drains from various process equipment in the petroleum and natural gas processing and storage facilities. Pressure relief of vapour/liquid and their disposal is not covered in this document for which OISD - 106 Standard on Pressure Relief and Disposal should be referred to.
3.0 CLASSIFICATION OF BLOWDOWN STREAMS
Before dealing with the design and operating practices for handling and disposal of blowdown, it is necessary to recognise the various blowdown streams generally handled in the petroleum industry. Based on the nature, frequency and other characteristics, these streams can be classified as below: 3.1 CLASSIFICATION BASED ON COMPOSITION Depending on the composition of blowdown streams these may be classified as:
3.1.1 Aqueous Blowdown: These streams contain water as the major constituent with small amounts of hydrocarbons. Such streams are usually encountered during the normal running of the units. These streams may be continuous or intermittent in nature. Some typical examples are:
Water draw-off from reflux drums Water-caustic (or other solvents) draw-off from
treating units Brine water draw-off from desalters and heater
treaters. Water draw-off from oil separators. Stripped water draw-off from sour water strippers Water draw-off from crude oil tanks and other
products tanks. Boiler blowdown 3.1.2 Hydrocarbon Blowdown These streams contain mainly the hydrocarbon fractions and may be encountered during normal operation, start-up, normal shutdown and emergency shutdown. These streams are usually intermittent in nature. Some typical examples are:
Intermittent draining of process equipment during normal operation.
Drains and vents/bleeders on pumps, pipelines, and manifolds
Sampling of products from equipment and piping Emptying of an equipment during planned
shutdown Emergency draining of a process equipment like
furnace, tower or vessel etc., Coker plant blowdown 3.1.3. Chemical Blowdown These streams contain aqueous solutions of chemicals and may have small quantities of hydrocarbons. Such streams may be continuous or intermittent in nature. Some typical examples are :
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Caustic drains from treating plants Reactor washes Effluents from water treatment plants Water draw-off from sour water strippers which
may contain sulphides, H2S, phenols, etc., Cooling Water blowdowns 3.2 CLASSIFICATION BASED ON TEMPERATURE Blowdown streams as classified above may further be grouped depending on the temperature. These streams may be hot at ambient temperature or be under cryogenic conditions. The hazards involved in handling them and the technique of handling and disposal of each one of such streams may be different. Some typical examples of such streams are: 3.2.1 Hot Blowdown
Furnace blowdown Coker plant blowdown Fractionating tower blowdown Hot heat exchanger/reboiler blowdown Samples of hot products Vents/drains from hot equipment Boiler blowdown-continuous and intermittent 3.2.2 Cryogenic Blowdown Drains, vents, and bleeders on equipment
operating at sub-zero temperatures like cold boxes, refrige-ration systems, etc.,
Drains, vents bleeders on flashing liquids. Drains, vents and bleeders on high pressure gas
transmission lines. 3.3 CLASSIFICATION BASED ON VOLATILITY
AND VISCOSITY Based on relative volatility, the blowdown streams may be classified as volatile or non-volatile. Similarly the may be viscous and congealing type or the free flowing type blowdown streams. Some typical examples are:
3.3.1 Volatile
Gas condensates LPG Gasoline and lighter napthas Hot-heavy hydrocarbons above their flash points.
3.3.2 Non-Volatile Kerosenes and heavier hydrocarbons below their
flash points. 3.3.3 Viscous/Congealing
Crude Oil Reduced Crude Oil Fuel Oil Bitumen Wax 4.0 SAFE HANDLING The facilities and procedures of safe handling of various blowdown streams will vary with the type and general grouping of these streams as listed earlier. Depending upon the type of blowdown, some typical handling facilities are recommended as under: 4.1 AQUEOUS BLOWDOWN Generally these blowdown streams consist of water with small amounts of hydrocarbons. Depending upon whether these streams are continuos or intermittent and the nature of hydrocarbons associated with them, the handling arrangements will vary as described in some typical examples below: 4.1.1 Aqueous Blowdown - Continuos in Nature: All aqueous blowdown streams from pressure vessels and which are continuous in nature, shall be provided with an automatic level indicator cum controller. In addition, high and low level alarms shall be provided in the concerned control room. Further, since majority of such services will be subject to corrosive and or dirty environment due to presence of large quantities of water, the level control valve should be provided with properly sized block and bypass valves for easy maintenance of the control valve. A typical scheme for handling such streams is depicted in Fig.1a. If the turned down ration of liquid blowdown is very wide as is the case vary often in oily-water is very wide as is he case very often in oily-water separators in oil fields, a multiple control valve assembly may be provided to take care of wide arrange of flow rates. However, if the continuous aqueous blowdown is form a vessel operating at atmospheric pressure, automatic level indicator-cum-controller may not be necessary. Instead a U-seal with a siphon breaker can be
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provided. Water draining from Barometric condenser drums and water seal vessels in flare headers are some typical examples of this arrangement. Refer Fig.1b for typical arrangement of such a scheme. All such blowdown streams consisting of mainly water from vessels which contain non-volatile hydro-carbons (which would not flash at ambient temperature can be disposed off to sewer. However, if blowdown stream consists of water from vessels, which contain volatile liquids like naphthas and lighter fractions, the same should be handled as discussed under 4.1.2 below.
4.1.2 Aqueous Blowdown likely to contain volatile Hydrocarbons.
In situations like light ends reflux drums, desalters, etc. where water is being withdrawn continuously from vessels containing lighter hydro-carbons, there is a possibility of dissolved hydro-carbons going to sewer. Further, a large quantity of lighter hydrocarbons can go to sewer in case the level control fails. This will create hazardous conditions. Under these situations, the water draw-off should be received in vapour disengaging drum. The vapours released form the top of this drum are lead to the close flare header and the free water from the bottom of the drum disposed to the sewer under an overflow pipe. The disengaging drum should be provide with high and low level alarms. A typical scheme for handling such streams is depicted in Fig.2. If there are more than one such sources of blowdown, a common disengaging drum can be considered. The overhead line from the disengaging drum should be properly sized after estimating the maximum amount of vapours released at the ambient conditions in the disengaging drum. 4.1.3 Boiler Blowdown - Continuous Continuous blowdown from a boiler/waste heat steam generator is usually at high temperature and pressure. Such streams should be handled carefully to avoid personnel hazards. A typical scheme of handling continuous boiler blowdown is depicted in Fig.3. If the boiler or stream generator operates at high pressure, the continuos blowdown streams are generally flashed into lower pressure steam system or heat exchanged for energy recovery before being disposed off safely. A stream of water is sprayed to the hot effluent water to
cool it before discharging into oily sewer or storm water drain.
4.1.4 Water Draw-off from Crude Oil Tanks This is an important routine operation intermittent in nature and can lead to lot of problems if not done properly. Though it is satisfactory to drain free water from the crude oil tanks into oily water sewer leading to waste treatment plant, draining of emulsion and wax. etc. into the oily sewer can lead to unsafe conditions in addition to increasing the load on the waste water treatment plant. it is recommended that the emulsion etc. from the crude tanks be received in a slop tank located into he crude tank farm area. The capacity of the slop tank should be adequate to handle the wax and BS & W likely to be present in a single largest crude tank. The slop tank should be provided with steam coils or alternative heating facilities to help break the emulsion. A typical scheme for handling such streams I depicted in Fig. 4 More than one crude tank can be served with a single slop tank located inside the crude tank(s) dyke area. However, if there are may large size crude oil tanks, more than one slop tank may be required to reduce piping and related problems After draining the free water from the crude tanks to oily water sewer, if oil water emulsion is observed, the draining should be lined up to the slop tank where the contents should be heated to the desired temperature to break the emulsion and allowed to settle for separating oil water phases. The water phase from the slop tank should be drained to oily water sewer and the oil content should be pumped back to the crude tanks by two slop oil pumps (1 operating + 1 standby) located outside the tank dyke. The slop tanks(s) should be provided with tempe-rature gauge at different elevations of the tank, a temperature. A level indicator and a high level alarm should be provided at the slop tank as shown in Fig.4. These indications and alarms should be taken to the control room or the local instrument panel as the case may be. It is essential that draining operations from the main crude tanks and the slop tank should be properly supervised by the operator to avoid large quantities of crude getting carried over into the sewer. 4.1.5 Other Aqueous Blowdown - Intermittent in
Nature Such blowdown streams are not continuous, but required to be handled during frequent draining or
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venting operations while the plant or facility is under normal operation. Typical examples of such systems are - water draining from storage vessels, product tanks and other process equipment etc. Usually, these streams consist of water, but may carry some traces of hydrocarbons. However, if not handled properly, large quantities of hydrocarbons can be drained alongwith water which can pose serious hazards. It is not necessary to provide automatic draining system in such situations. The blowdown, drain or vent lines should be provided with manually operated block valve(s) preferably double valves to enhance the integrity of isolation. It is essential that intermittent draining of water from process equipment and storage vessels be always attended and no draining or venting should be allowed to continue without an operator attending to it. A typical arrangement of draining water from LPG storage vessels is depicted in Fig.5.
4.2 HYDROCARBON BLOWDOWN STREAMS These blowdown streams contain mainly hydro-carbons. Most of these streams are intermittent in nature and are required to be handled during frequent draining or venting operations while the plant or facility is under operation. Such streams are also required to be handled during a planned or emergency shutdown or start up of the plant/facilities. It is essential that such operations be always attended and no draining or venting should be allowed to continue without operator attending to it. The blowdown drain or vent lines should be provided with double valves to enhance the integrity of isolation. 4.2.1 Draining/Venting from Hot Pumps: During start up of operation, it is required to drain or vent/bleed hydrocarbon pumps occasionally.. If not handled properly, venting and draining of pumps handling hot products can lead to hazardous conditions due to autoignition. This risk is higher especially for hot and heavy products like fuel oil etc. whose auto-ignition temperatures are usually lower (250 Deg - 300 Deg. C range). Autoignition temperatures of products generally reduce as the molecular weight of the product increases. Vent and drains of each hot pump should be provided with small sample cooler (located adjacent to pup at operating elevation) so that the product is cooled
before being released to sewer. Whenever venting or draining of such pumps is undertake, the cooler is lined up and water flow through cooler established before opening the pump drain or vent line. Depending upon the location, a common cooler can be provided for more than one pump. A typical scheme for handling such streams is depicted in Fig.6 a low pressure (l.P) steam connection is provided to the sample cooler to heat the content of the cooler to a temperature commensurate with the pour point of the product. This will ensure that the product coil does not congeal while handling high pour point - waxy products. 4.2.2 Draining and venting from LPG/Cryogenic
Product Pumps: Draining and venting of pumps handling LPG and other cryogenic products should also be done very carefully. The products coming out of these pumps would flash immediately giving large quantities of vapours and also create low temperature in the vent and drain piping. Such pumps should be drained/vented to flare or to a closed vent header if there is no flare in the facility. Additionally drain connection to oily sewer with valve and cap should be provided to facilitate draining of water or heavy ends. A typical scheme for handling such streams is depicted in Fig.7. Double valves should be provided on the vent and drain header lines as shown in the sketch - one isolating valve and the other globe/needle valve for control. While handling cryogenic liquids, the temperature in the vent/drain lines may drop down due to flashing of liquid form higher pressure to lower pressure. This phenomena should be considered while selecting the materials for such piping. 4.2.3 Draining of Equipment for Shutdown: Various equipment are required to be drained free of hydrocarbons and other hazardous materials during shutdown for maintenance or inspection. For infrequently used connections normally associated with equipment maintenance, the drain lines should be provided with block valves and the connection should be either plugged capped, or blinded for integrity of isolation. Drain lines carrying such streams from process equipment having large inventories should be piped to the unit blowdown drum(s) or a pump out header for the safety and recovery of the hydrocarbon
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product. Fig.10 depicts a typical arrangement of a unit blowdown drum. 4.2.4 Sample Points: Small quantities of hydrocarbons are drained while taking samples of various products which pose hazards if not handles carefully. It is recommended to locate all the sample points at one convenient location in the unit and lay a collection tray below the sample points. Sampling lines should be small in size preferably and as short as possible to minimise the amount getting drained while taking the representative samples. Each sampling line should be provided with tow valves, one isolating valve and the other regulating valve. Hot products should be provided with suitable sample coolers. Hydrocarbons drained and collected in the tray should lead to a drum, from where the material should be disposed off to a safe location manually. Alternatively, the tray should be piped to the blowdown drum if the same exists. See Fig.8 for a typical sample points installation. 4.2.5 Gas-Condensate Blowdown: When the blowdown stream consists of condensate drawn-off from gas-liquids separators inter/after condensers/coolers or compressors etc. the blowdown line should be provided with automatic shutdown valve actuated on low level in the separator to avoid gas blow-by in the downstream liquid system which usually operates at lower pressure. A typical scheme for handling such streams is depicted in Fig.9
4.3 EMERGENCY BLOWDOWN In certain emergency situation, it may be required to drain or dump the contents of a process vessel or equipment to a safer location. Such blowdown streams should be handled automatically and preferably from a remote location. Some typical examples are: 4.3.1 Blowdown from LPG and other cryogenic
vessels:
During a fire emergency, vessels containing LPG and other such products pose additional hazards because of BLEVE (Boiling Liquid Expanding Vapour Explosion) effect. The fire increases the internal pressure and weakens the vessel until the vessel can no longer contain the pressure. The vessel the ruptures violently, with its parts propelled to great distances. The
released liquid flashes often resulting in a large fireball. The fireball can cause vary widespread damage due to flame contact. Although the fire ball lasts only a few seconds, its effects can be devastate. Pressure vessels containing LPG or other such liquids located in the process areas have higher risks to BLEVE as compared to storage vessels in the offsite area. In a fire situation, therefore, it is desirable to empty out the liquid inventory of the process vessels containing LPG etc. to a safer location. Such vessels therefore should be connected to a blowdown drum thorough adequately sized blowdown line so that the liquid content of the vessels can be drained into the blowdown drum in the shortest possible time (say 5-10 minutes). The blowdown line from the vessel should be provided with a tight shut off shutdown valve remotely operated from the control room. Similar blowdown for large size offsite storage vessels is usually not practicable. However, facilities for transferring inventory of LPG etc. from one storage vessels to other should be considered. The blowdown drum mentioned above can usually be the common light oil blowdown drum located at one corner of the unit and connected to the flare header. The stock from the blowdown drum can be pumped for reprocessing or recycle etc. The capacity of the drum should be adequate to receive the total contents of a single process vessel containing LPG or similar materials. This drum should not be combined with hot blowdown drum which received heavy and fouling type products. A typical arrangement of such a blowdown drum is depicted in Fig.11. Refer OISD-106, Standard on Pressure Relief and Disposal System for more details on blowdown drums. 4.3.2 Furnace Blowdown: Some furnaces are provided with emergency drum system to drain out the hydrocarbon contents of the heating coils. Such blowdown streams should be piped into a header terminating into a quench drum where the hot blowdown is cooled to a safe temperature and disposed off suitably. Fig.12 depicts a typical arrangement of a quench drum. Refer OISD-106, Standard on Pressure Relief and Disposal System for more details of quench drums.
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4.4 CHEMICAL BLOWDOWN Like aqueous blowdown streams, chemical blowdown streams and drains may be continuous or intermittent. These should be handles with care to avoid accidents. The design and operating philoso-phies for handling continuous and intermittent aqueous blowdown streams also. As discussed earlier under Aqueous Blowdown, the continuous chemical blowdown streams should be controlled automatically under level of the vessel, and where there is a possibility of hydrocarbon phase coming out with the chemical, disengaging drums should be considered. The integrity of isolation of inter-mittent blowdown streams by double valves, blinds or caps should be ensured. Refer section 5.3.3 for disposal of chemical blowdown streams. 5.0 DISPOSAL OF BLOWDOWN The various blowdown stream handled above should be disposed off safely to their respective destinations. Selection of disposal system will depend on the type of blowdown stream. Disposal system may be closed pressure system or gravity sewers. 5.1 CLOSED PRESSURED SYSTEM All hydrocarbon blowdown streams free of water and chemicals etc. which re drained from various vessels and equipment are generally received in a closed blowdown drum at one corner of the unit. The blowdown streams are pumped out from this closed blowdown drum by means of pump(s0 and routed to feed tanks, slop tanks or downgraded to other products, as the case may be, by suitably designed closed piping system. These streams should not be put into sewers from safety and loss considerations. A typical scheme of such system is depicted in Fig. 10. The blowdown drum can be a common drum for more than one unit and located underground at a convenient location and vent to atmosphere so that the different vessels and equipment can be drained to the drum under gravity head through closed piping system. The vent line from the drum should be provided with snuffing stream. The pumpout pump is usually a vertical submersible pump capable of starting and stopping automatically on high and low liquid levels in the drum.
5.2 SEWER DRAIN SYSTEM Gravity sewer drains are extensively used in the petroleum industry for the disposal of various waste and blowdown streams containing oily, chemical and other wastes etc. Generally such items of plant or facility get low profile, but the importance of their good design and operating practices cannot be under-estimated from the safety and loss prevention point of view. Important guidelines for the design and operation sewer system are given below: 5.3 TYPE OF SEWERS Depending upon the nature of waste streams, the sewers should be segregated as below:- 5.3.1 Oily Water Sewers: The following aqueous streams generally have some oil content carried by them. These streams should be routed to oily water sewer system:- Oily water form process unit - Reflux drums,
separators, disengaging drums, etc.
Gland and seal cooling/quenching water from pumps/compressors etc.
Brine water/sour water.
Process wash water.
Oily water from hot wells.
Floor and paving drains in oily drums
Oil + water overflow from quench drums.
Once through water from barometric condensers
Hydrocarbon tanks drains
Sample point drains.
Oily system Condensate if no condensate recovery and treatment facilities exist.
Process cooling tower blowdown. Captive power plant cooling tower blowdown streams are not likely to contain oil and therefore, may be routed to storm sewer.
5.3.2 Storm Water Sewers: Waste water which has no oil and other objectionable material can be handled by storm water system. This system includes closed conduits and open
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ditches. Following streams should be routed to storm water sewer system.
-- Rainfall from clean areas -- Boiler blowdown -- Steam condensate -- Tank Diked area drains (alternate routing as the
same should also be routed to oily water system) -- Treated water effluent from DM water plants if PH
and COD are Okay. 5.3.3 Chemical Sewers: Sewers carrying drains/effluent wastes containing chemicals should be segregated form other sewers since these streams often require different treatment than oily water streams before final disposal. Such streams are also generally corrosive and may required special materials of construction for the sewers. Since the nature of treatment may be different for different t chemicals, it is necessary that the quantities, type and compositions of all such streams likely to be handled in the process areas, chemical storage, handling and dilution etc. should be thoroughly studied to decide their segregation and treatment. If chemical waste affect only oxygen demand and solid content of the effluent, these can be discharged to the oily water system downstream of primary oil separation but upstream of biological treatment. However, when pH, odor, taste, color, trubidity etc. are affected, appropriate special treatment should be considered at suitable stage before biological treatment. Since each chemical may require a specific treatment, it is difficult to generalise the treatment facilities here. Detailed guidelines for effluent treatment facilities are therefore not included in the scope of this document. 5.3.4 Sanitary Water Sewers: Sanitary water sewers should collect wasted from toilet facilities and convey these to a treatment system in the complex, municipal sewer system or to a septic tank depending upon the local condition. The treated sanitary effluent may be discharged to the treated oily water system, to the storm water system or independently to the desired disposal point.
5.4 BASIC DESIGN GUIDELINES FOR OILY-WATER SEWERS
5.4.1 Layout of Oily Sewers: i) REQUIREMENT OF SEALED MANHOLES:
The layout of most oily water sewers consists of a trunk sewer with branch connected to process units, diked areas and other work areas. The main trunk sewer should be isolated from branch sewers by sealed manholes so that the vapours cannot back up into processing areas. Seals play a vital role in maintaining safe operation of a sewer system. Sealed manholes shall be provided on oily sewers at the following locations: (a) Process Units On unit oily sewer at unit battery limit so that unit
area is cut of from any fire in offsite areas or vice versa.
Within process units, sealed manholes shall be provided in such locations so that each sub-unit within the unit is isolated from other areas. Where such demarcation is difficult, one sealed manhole for every 30m length of unit sewer shall be provided.
At change of direction of sewer line.
At entry of branch line to manhole. b) Offsite Areas At connection manholes of branch sewers to main
sewers to isolate each facility contributing oily waste from the risk of fire. Such connections may be from tank farms, offsite pump stations, oil loading/unloading areas, railway gantry areas, compressor house, etc. etc.
On main sewer lines at suitable points so as to provide dire barriers for segregation of vital hazardous installations from each other. In general sealed manholes on main sewer lines shall be provided at intervals of not more than 300 meters.
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ii) TYPE OF SEALED MANHOLES The following fire sealed manholes should be provided:
(a) Single compartment sealed manholes with bent pipes seal type shall be used for CS pipes upto diameter 16.
(b) Double compartment type sealed manholes shall be provided for CS pipes greater than 16 dia and for all sizes of RCC pipes.
(c) A minimum of 150 mm fire seal shall be provided for both single and double compartment type manholes.
Fig. 13 & 14 depict typical sealed type manholes Single compartment and double compartment type. Sewers, in general, are designed for gravity flow. In a tightly sealed system, a rise in water level would reduce the vapour space causing obstruction to flow. Vents should therefore be installed on the manhole to maintain atmospheric pressure in the sewer and to release vapours to safe locations. Care should be taken to avoid vents near furnaces etc. In critical locations, the vent pipes should be provided with steam snuffing connections. 5.4.2 Capacity The branch sewers from processing areas should be designed for the greater of the following two situations
Rainfall plus process waste water with the sewer flowing at 2/3 of full depth.
Process waste water plus expected fire water run off with sewers running full.
The design capacity of the trunk sewer should depend on the cumulative amount of spend cooling water and condensate from various processing areas and the storm drainage from the paved areas and the largest tank dyke area. Trunk sewers generally drain large water sheds, therefore, fire water flows are not governing for their sizing. However, this should be checked back. A minimum velocity (at half full or running full) of 0.6 m/sec should be maintained in the branch and trunk sewers to avoid settling of solids. If the water consists of large quantities of sediments, the minimum velocity
should be increased to 1m/sec. The maximum velocity should be limited to 2.4 m/sec. The slope of sewers should be decided based on the sewer size. 5.4.3 Material of Construction Cast iron and hume pipe are usually the best materials for gravity drains. If salt water is likely to be used in the plant, it is advisable to use pipes which are coated with cement from inside. 5.5 SPECIAL SEWERS Because of special nature of some effluent stream-high pour point, wax content, viscosity and other fouling characteristics as is the case with asphalts, waxy lube/intermediates and other similar products, their drains should not be routed to oily water sewers directly as there is danger of the entire sewer system getting choked. Such product drains should be handled separately. Some specific cases are mentioned below: 5.5.1 Asphalt Drains All equipment and areas where effluent streams containing asphaltic material is likely to be handled should be segregated from other areas and bunded by a 4-6 high concrete wall having a baffle at one end. A small water stream should always be kept flowing in the bund. The water should overflow into a catchpit having two compartments from where the water should overflow into the oily water sewer. Asphaltic material which gets trapped in the catchpit should be occasionally scrapped off manually by isolating one compartment. Provision should also be made for steaming the catchpit in case required. 5.5.2 Waxy Drains All equipment and areas where effluent streams containing waxy materials are likely to be handled should be likewise segregated from other areas and bunded by 4-6 high concrete wall having a baffle at one end. The water should overflow from the baffle into the oily water sewer. The wax will get deposited within the bund from where it should be scrapped off occasionally. 5.5.3 Drains at Remote Locations Sometimes because of small capacity of an installation and or its remote location, it may not be
9 OISD-109
feasible to have elaborate oily water sewer system and treatment facilities. Under such situations, local sumps should be provided for collection of oily water effluents from such an installation. Oil sumps should be designed as miniature oil separators with outlet baffles to retain oil. Suitable skimming arrangement should be provided to withdraw oil to a storage pit, from where it can be pumped out to slop, tank or barrels etc. Oil sumps however are not very efficient for compete removal of oil from waste. Alternatively, a Tilted plate type of oil-water separator can be specified. Oil removal in such separators is very good and the treated wasted water almost oil free. If waxy or fouling type of products are envisaged in the waste water, a grid screen or a filter should be used upstream of these separators.
6.0 REFERENCES: 1. Design guide to Refinery Sewers, J.D. Brown and
G.T. Shannon An API Survey.
2. API-521, 1982 Guide for Pressure Relieving and Depressurising Systems.
3. Safety Digest of Lessons Learned, Section 3, API Publication, 758, 1980.
4. Design for Process Safety, Hydrocarbon Processing, December, 1985.
5. OISD Standard 106 on Pressure Relief and Disposal System.
OISD-109 25
Note: TI point shown in the drawing below should be located on the downstream of control valve. Note 1
FIG
. 1a
AQ
UEO
US
BLO
WD
OW
N
CO
NTI
NU
OU
S IN
NA
TUR
E
OISD-109
26
Fig.
1b
Aqu
eous
blo
wdo
wn
C
ontin
uous
in N
atur
e
OISD-109 27
FIG
. 2
AQ
UEO
US
BLO
WD
OW
N
LIK
ELY
TO C
ON
TAIN
VO
LATI
LE H
YDR
OC
AR
BO
NS
OISD-109
28
FIG. 3 CONTINUOUS BOILER BLOWDOWN
OISD-109 29
FIG
. 4
WA
TER
DR
AW
O
FF F
RO
M C
RU
DE
TAN
KS
OISD-109
30
FIG
. 5
DR
AIN
ING
OF
LPG
VES
SELS
OISD-109 31
FIG. 6 DRAINING/VENTING OF HOT PUMPS
OISD-109
32
FIG
. 7
DR
AIN
ING
/VEN
TIN
GO
FC
OLD
PUM
PS
OISD-109 33
FIG
. 8
SAM
PLE
POIN
TS
OISD-109
34
OISD-109 35
FIG
. 10
BLO
WD
OW
ND
RU
M
OISD-109
36
OISD-109 37
OISD-109 25
OISD-109
26
OISD-109 27
NOTES
OISD-109
28
NOTES
OISD-109 29
NOTES
OISD-109
30
NOTES
