PETRONAS TECHNICAL STANDARDS

DESIGN AND ENGINEERING PRACTICE

MANUAL (SM)

DEOILING INDUSTRIAL

WASTEWATER DESIGN AND

OPERATION OF C.P.I.

PTS 20.065

MAY 1981

PREFACE

PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication,of PETRONAS OPUs/Divisions.

They are based on the experience acquired during the involvement with the design, construction,operation and maintenance of processing units and facilities. Where appropriate they are basedon, or reference is made to, national and international standards and codes of practice.

The objective is to set the recommended standard for good technical practice to be applied byPETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemicalplants, marketing facilities or any other such facility, and thereby to achieve maximum technicaland economic benefit from standardisation.

The information set forth in these publications is provided to users for their consideration anddecision to implement. This is of particular importance where PTS may not cover everyrequirement or diversity of condition at each locality. The system of PTS is expected to besufficiently flexible to allow individual operating units to adapt the information set forth in PTS totheir own environment and requirements.

When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for thequality of work and the attainment of the required design and engineering standards. Inparticular, for those requirements not specifically covered, the Principal will expect them to followthose design and engineering practices which will achieve the same level of integrity as reflectedin the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from hisown responsibility, consult the Principal or its technical advisor.

The right to use PTS rests with three categories of users :

1) PETRONAS and its affiliates.2) Other parties who are authorised to use PTS subject to appropriate contractual

arrangements.3) Contractors/subcontractors and Manufacturers/Suppliers under a contract with

users referred to under 1) and 2) which requires that tenders for projects,materials supplied or - generally - work performed on behalf of the said userscomply with the relevant standards.

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CONTENTS

INTRODUCTION

Modifications to the CPI

Example calculation of plate pack

(a) Oil separation

(b) Type of flow between plates

(c) Stability of flow between plates

Relation overweight of oil and depth oil/waterinterface in CPI

Diagram Relation Particle size/settling velocity

Main drawings (not to scale)

Corrugated Plate Interceptor:

S 14.040 Construction - General Arrangement

S 14.041 Start-up and operation (text)

S 14.042 Assembly of Details 1 up to 11

S 14.043 Assembly of Details 12 up to 16

S 14.044 Assembly of Details 17 - 18 and 19

S 14.045 Flow Distribution Baffle

S 14.046 General Arrangement one plate assembly

S 14.047 Multiple Bay Arrangement

Appendix 1. Prefabricated steel CPI basins.

Appendix 2. CPI with inlet weir:

S 14.021 Construction, Start-up, Operation

S 14.022 General arrangement (2 bays)

S 14.024 Design principles

Detail drawings

T 960299 Assembly of steel parts (not on scale)

T 960300 Detail weir

T 960301 Detail skimmer

T 960302 Connection flanges for skimmer

T 960303 Trash pan

T 960304 Overflow outlet pipes

T 960305 Trash rack

T 960306 Detail suspension sludge suction pipe

T 960307 Detail hand railing

T 960327 Flow distribution baffle

INTRODUCTION

The basic treatment for de-oiling refinery waste water is done in gravity-type oil interceptors. Theconventional oil interceptor consists of a basin through which the waste water flows so slowly that oilglobules have time to rise to the surface. At the end of the basin, the floating oil is retained by a baffleand skimmed off.

By subdividing an interceptor basin into longitudinal channels with parallel plates, its separatingcapacity can be increased. If for instance the basin is subdivided into ten layers, the maximum path(raising height) which an oil globule has to travel before reaching a coalescing surface is reduced toone tenth of the depth of the water in the basin.

An additional advantage is that, at flow velocities such as used in conventional interceptors, there islaminar flow between the plates. Laminar flow is ideal for gravity-type separation and it cannot beobtained in the large cross section of conventional interceptors.

Good results have been obtained with the PPI (Parallel Plate Interceptor), which has found wideapplication. The separation compartment of the PPI consists of a rectangular basin in which plates areinserted at an angle of 45° in cross direction and parallel to the horizontal bottom in the longitudinaldirection.

Ten years' experience with the PPI has led to the development of the CPI (Corrugated PlateInterceptor).The latter interceptor is cheaper, it overcomes some operational difficulties experiencedwith the PPI and it is better suited for a corrosion or acid proof construction.

This report gives information on the design and operation of the CPI. A set of "to scale" drawings canbe requested from PETRONAS.

MODIFICATIONS TO THE CPI

Since its introduction some 13 years ago, development of the CPI has continued. Years of experienceand a good feedback from CPI users has resulted in the introduction of a number of modifications, tothe original version of the CPI, which result in simplified construction, operation and maintenance andthus an improved performance of the CPI.

It should be noted however, that under certain circumstances application of the original version of theCPI is still valid. The modifications and the motivation for the introduction of these modifications aredescribed below:

Theoretically speaking the weir is possibly not the most ideal inlet device for a CPI, but it was adoptedsince it displays certain advantages:

1. It is a simple way to distribute flow equally over several basins, especially for large batteriesof CPI's, and to allow for further settling of the structure.

2. It prevents accumulated oil from flowing back to the drainage system.

3. The difference in water levels on both sides of the weir is so small that it can be considered tobe semi- submerged weir, thus minimizing dispersion (this compares favorably with normalweirs).

Due to changes in operating policies of the CPI, by refineries, it is now possible to introducemodifications to the CPI inlet. The original policy was to generate a thick oil layer in order to skin off oilfree water (but at the same time accepting some emulsion forming at the oil/water interface and thuspossibly more pollution).

For ease of operation, however, skimming is now carried out on a more frequent basis (thus acceptingwater-contaminated oil in the slops system). This change in operating policy has also resulted in aimprovement in effluent quality.

The inlet weir is therefore no longer strictly required:

(a) to build up an oil layer;

or

(b) to prevent oil from flowing back.

By omitting the weir, the velocity gradients are virtually eliminated, and, by turning the flow distributionbaffle to a vertical position, a regular flow will enter the CPI pack compartment.

The stilling basin is now obsolete.

Where (a) and (b) are of paramount importance, the original version of the CPI may be advisable anddetails are therefore still available (see Appendix 2.).

To facilitate skimming off, the fixed oil skim pipe has now become movable. The width of the inletchannel is indicated as being variable; for example, if the flow is composed of process water and run-off water, sand and slugs of oil can be expected. A wide inlet channel is then favorable. The sameholds when large slugs of oil are expected. The inlet channel can then serve as a kind of pre-separator. For instance, if the flow is composed of process water and only some run-off water, thewidth of the inlet channel can be minimized.

In the outlet basin the installation of fixed pipes for "high"-pressure water jets is suggested. It will bevery simple during cleaning operations to clean the basin after stabilized sludge has been loosenedby the water jets.

Where fouling of the plate packs can be expected due to high concentrations of suspended solids inthe effluent stream the newly developed comb. plate packs are recommended instead of the gutterplate packs. This eases maintenance and reduces fouling of the plate packs.

Where odour and/or airborne sand and dust cause a major problem corrosion-proof lightweighthoods/covers are available.

In some cases it may be desirable to construct the CPI above ground level. A prefabricated steelbasin could then be considered as an alternative; especially in the following circumstances:

- time is a limiting factor.

- mobility is required; e.g. re-use on other site.

- desludging deserves special attention.

- prefabrication may solve economic and organisational problem.

An example of a steel basin is included as Appendix 1.

It should not be forgotten that gravity separators are relatively cheap because of their simplicity. Ifdemands are for less supervision and less manpower, the increased sophistication entailed has to bepaid for. This may be a topic of discussion between designers and users.

For many years the only treatment existing at refineries was gravity separation of oil and water. Theeffluent quality depended, therefore, on the efficiency of the separators. Improvement ofhousekeeping, maintenance of the process plant equipment and good control of the separators had agreat influence on this efficiency. However, owing to the tendency to reduce manpower, these factorshave tended to deteriorate. Moreover, the fact that secondary treatment is becoming are requirementat most refineries is additionally contributing to the neglect of these factors. Nevertheless, theefficiency of the sophisticated secondary treatment still depends on the influent quality and, hence, onthe primary treatment.

This report gives information on the design and operation of the CPI. A set of "to scale" drawings canbe requested from PETRONAS

Example calculation of plate pack (outside dimension 1.00 x 1.00 x 1.75 m)

a) Oil separation

Interval of plates: d = 20 mmMaximum rising height: H = d√2 = 28 mmTotal length of plates in flow direction: Ltotal = 1.50 m

Effective length of plates = Ltotal minus length required for in and outflow: L eff = 1.25 m

Effective cross section: F = 1.00 x 0.90 = 0.90 m2

Throughput: Q = 30 m3/h (normal load on one plate pack)

Flow velocity: V w = QF =

30 100 9 3600

9 263x

xmm

.. / sec=

Overflow rate:

Vh x V

Lx

rw= =

28 9 261500

.= 0.2 mm/sec which means

that oil globules with a rising velocity equal to or more than 0.2 mm/sec are completelyintercepted.

b) Type of flow between plates

Interval of plates: d = 20 mmEffective cross section: F = 0.9 m2Throughput: Q = 60 m3/hr (maximum load on one plate pack)

Flow velocity: V w = QF

xx

mm= =60 10

0 9 360018 5

3

.. / sec

Hydraulic Radius = Area between plates

Wetted perimeter

2 =

20 10002000

10x

mm=

Kinematic viscosity at 20° C water temperature:

υ = 1.0 x 10-6 m2/sec

Reynolds No. : Re = V R x x. . . .

.υ=

0 0185 0 01 1010

6

180 < 400 which means that at 20° C water temperature there is laminar flow betweenthe plates at a throughput of 60 m3/h.

Relation: Overheight of oil and depth oil/water interface in CPI

∆h hS S

So ww o

o

=−( )

in which :

∆ ho = overheight of oil above water level at outlet weir

hw = depth of oil/water interface (maximum 70 cm)

Sw = specific density of water

So = specific density of oil

It is noted that under normal conditions, the overflow height of oil above the edge of theskimmer is less than the overflow height of water above the edge of the outlet weir,because less oil than water will enter the CPI.

Assuming the overflow heights are the same, then hw max.will be reached when So = 940 and Sw = 1000

For oils with So > 940, the difference in level of the oil

skimmer and the outlet weir (on drawing 5 cm) can be decreased with the disadvantage thatat the maximum load (60 m3/h per plate pack) over a long period, all oil may be skimmed offand water may enter the oil sump.

c) Stability of flow between the plates

For stable flow: Froude No. : F = V

R g

25110

..> −

(in which g = acceleration caused by the force of gravity)

∴ V2 > 10-5 X R x g = 10-5 x 0.01 x 9.81 x 106 = 0.981 mm2/sec2

∴ V > 0.99 mm/sec

∴ Q > V X F = 0 99 0 9 3600

10003 2 3

. .. /

x xm h=

Consequently for a plate interval of 20mm, there is stable flow between the plates at athroughput of more than 3.2 m3/h.

For a plate interval of 39 mm there is stable flow at a throughput of more than 4.5 m3/h.

APPENDIX 2

CPI WITH INLET WEIR