4 replies to this topic

[kkala]

I work in an engineering company, having undertaken the basic design of one slops floating roof tank to store white and black petroleum off spec products, as per Client request. These slops can be anything from aviation kerosene (e.g. JP4) to fuel oil, as well as water and a black slurry of about 1.06 specific gravity. Max operating temperature for ingoing white slops is 40 0C, for black slops 60 0C. Conditions are such that the tank contains a lot of above slops at a time (e.g. JP4, diesel, heavy gas oil, fuel oil, black slurry), like a buffer tank for all slops. Stored liquid is gradually pumped to the proper refinery units.
Black slops flow rate is limited, so that stored quantity never exceeds tank level of 2 m from bottom according to Client, who also requires a steam (3.4 barg) coil close to tank bottom to maintain black slops temperature at 60 0C (viscosity low enough for transfer).
This tank (net capacity ca 14000 m3) will normally contain several layers of liquids from bottom up: mentioned black slurry (not always), water (height not negligible), black and white hydrocarbon liquids of s.g.<1.0. Slops will be carried out by pump through either a floating suction or a normal suction (the latter near tank bottom). Sketch can be seen in attached slops.doc.
Client has excluded naphthas and lighter products, so that total vapor pressure of stored liquid should not exceed 13 psia (limit for floating roof tanks) at 60 0C.
The problem is that an engineering practice document says that storing (and heating) black and white slops together in the same tank is not recommended, due to the risk of boilover / frothover. It is a safety issue. Any advise on the matter or on what is industrially applied for slops would be appreciated. Counter argument from Client is that there are crude floating roof tanks with heating coils .
It is also noted that the floating roof can be stabilized at ~1.1 m from bottom for maintenance, so there should be in this space between: steam coil (plus spare) + series of thermocouples (depth ~1 m from side wall) + probably 2 lateral agitators + nozzles + level sensors etc. Thus black slops measured temperature seems hard to be representative.

Edited by kkala, 13 February 2010 - 06:15 AM.

[kkala]

I have not managed to attach mentioned slops.doc file by editing my question posted a few time ago, so I do so sending this "reply" with slops.doc attached. Sorry for the inconvenience.
Please also note bold was used in the question just to distinguish question from data, since status seemed a bit confusing.
This "slops tank" design has stopped by the Client, but now it will start again.
Best regards to all

Attached Files

•  slops.doc   56KB   124 downloads

[kkala]

Since the first post, subject has been investigated by another colleague too. The conclusion is it is not recommended to mix black and white slops due to the risk of boil over / froth over of the resulting liquid in its downstream storage tank.
Slops most often contain water settling on tank bottom (risk of boil over). Slops of substantially different density tend to get stratified in layers in the tank (risk of froth over). These hazards may occur not only during a fire, but also during heating by a steam coil which can get (α)steam leakage (β) loss of temperature control; or even by a hot stream into the common storage tank, e.g. asphalt.
Therefore today's practices instruct "positive segregation" between black and white slops, separate storage, no heating in thanks containing white slops. Common header for all (black & white) slops does not deem safe enough.
Hot streams similar to asphalt or fuel oil should be cooled below 93 0C (to avoid boil over) upstream storage tank. If this is not feasible (e.g. PSV discharge), a dedicated storage of adequate capacity for Operator to intervene and stop the hot stream (usually 20 min) could be a way out.
White streams of high temperature should be cooled to a vapor pressure lower than the limit for floating roof tanks (about 12-13 psia), if stored in such tanks.

Traced relevant accidents are as follows (hazop study is recommended on slop systems).
(α)Refinery, 1968, tank with steam coil storing "mixed" slops: 50-100 tonnes of released hydrocarbon vapors caused a vapor cloud explosion, killing 2 and injuring 85 people. It is believed release was due to water oil emulsion separated at 100 oC.
(β)Vacuum gas oil of 204 oC was transferred to a cold slops tank (cone roof). About 10 tonnes of hydrocarbons escaped out, either as vapors or as liquid into the dike area. No fire occurred.
Supposedly more such accidents have occurred.

Useful references:
http://en.wikipedia.org/wiki/Boilover
http://www.bitumenuk..._2010_final.pdf
http://www.fireworld..._water_1107.php
http://upcommons.upc...accidental.pdf'> http://upcommons.upc.edu/e-prints/bitstream/2117/7373/1/accidental.pdf

Note: A crude tank seems similar to a tank storing black and white slops. Crude tanks can have steam coils. So why tanks storing black and white slops are not recommended by practices? Probably because nothing else can be done for crude tanks, contrary to slops tanks. Is there any other explanation?

Edited by kkala, 22 April 2011 - 03:35 PM.

[sheiko]

I work in an engineering company, having undertaken the basic design of one slops floating roof tank to store white and black petroleum off spec products, as per Client request.

Please excuse my ignorance but, could you please explain what is meant by "white" and "black" slops? I have also heard of "wet" and "dry" slops, but I am not really familiar with these english terms...

Edited by sheiko, 23 April 2011 - 09:50 PM.

[kkala]

I work in an engineering company, having undertaken the basic design of one slops floating roof tank to store white and black petroleum off spec products, as per Client request.

Please excuse my ignorance but, could you please explain what is meant by "white" and "black" slops? I have also heard of "wet" and "dry" slops, but I am not really familiar with these english terms...

Black slops are off spec black petroleum fractions or products, such as atmospheric residue, fuel oil, asphalt; also called heavy slops, they require heating during storage and handling. Vacuum gas oil (brown color), handled in heat traced lines, is usually classified in this category.
White slops are off spec white petroleum fractions or products, such as naptha, kerosene, jet fuel; also called light slops, they are handled and stored at ambient temperature.
So specific gravity limits for products can (approximately) be: 0.9 & higher for black slops, 0.8 & lower for white slops. Intermediate values (i.e. 0.8-0.9, e.g. diesel, light gas oil) concern a "grey" area (relevant slops are sometimes considered as light ones). Storage & handling of these "intermediate" (term nowhere found) slops, as well as mixing with other slop categories, had better be considered through a HAZOP study (generally recommended by practices for any slop system). Slops of crude are mixtures of all mentioned slops, so they cannot be classified as above.
Terms heavy / light (or black / white) slops do not seem to have been strictly defined, so differences of understanding may exist from refinery to refinery or place to place. However above hopefully complies with this thread. Clarification should have been tried by me earlier.
Wet / dry slops can be met at http://www.cheresour...9-what-is-slop/. My impression is that today refineries try to recover any oil contained in slops, unless sent to treatment as waste water.

Edited by kkala, 24 April 2011 - 05:19 AM.