Tank Mixer Test

Two Plenty Swivel Angle Mixers were installed in Tank No. 1141, Terminal South, Ras Tanura.

 

This Tank contained approximately two feet of sludge ( 10,000 Bbls. ) which according to the laboratory analysis was: -

The Mixers were installed to prove their effectiveness in suspending this sludge in the crude oil and hence reducing the volume accumulated sludge.

The tests were a complete success. After only 48 hours running, the Mixers reduced the sludge from two feet to less than one inch.

Having cleaned the tank bottom by suspending the sludge in the oil, it was then possible to drain some 3,600 Bbls. of water which had been held in the sludge. The water is held in the sludge in the form of partial emulsions, together with some "free" water.

During the 48 hours mixing time, all the water was suspended in the form of an emulsion and was, therefore, reluctant to settle. A demulsifier - TRETOLITE RP-968 was introduced at the rate of 30 ppm. to break the emulsion and allow the water to settle out quickly.

This also was a very successful operation, as within 24 hours from the introduction of the Demulsifier and mixing, large quantities of water were drained from the tank.

1) One Mixer proved ineffective in suspending the B.S. & W. in this 360,000 Bbls. Tank.

2) The two Swivel Angle Mixers recommended proved very effective in suspending B.S. & W.

3) The two Swivel Angle Mixers proved effective in blending TRETOLITE with ARABIAN LIGHT crude oil.

4) TRETOLITE RP-968 proved effective as a demulsifier for water in ARABIAN LIGHT crude oil.

5) Water in crude oil Tanks is held by the sludge in partial emulsions and trapped "free" water. After the first mixing operation, 600 Bbls. of water was drained off and the B.S. & W. analysis of the oil samples showed NO WATER. Yet after further mixing at other angles, a further 3,000 Bbls. of water was drained off.

6) Mixers proved successful in releasing water trapped in accumulated sludge on the bottom of crude oil Tanks.

7) It is possible to drain most of the water from a crude oil Tank, provided the Tank bottom is kept clean and free from sludge.

8) The Mixer manway orientation was not ideal (i.e. opposite outlet connection). however, the Mixers still proved effective when used in the available manways.

9) The recommended installed horsepower (100 h.p.) proved correct in 376,000 Bbls. Tank capacity.

10) Insufficient installed horsepower is a complete waste of time and money, as proved by the results of trying to use just one Mixer (50 h.p.) at the start of the test. The minimum horsepower required for B.S. & W. duties is approximately 0.25 h.p. for 1000 Bbls. tank capacity.

In view of the unqualified success of the Mixer test, the following recommendations are made.

These recommendations fall into two categories - PREVENTION AND CURE.

The first recommendation must be that prevention is better than cure.

PREVENTION OF SLUDGE ACCUMULATION

1) PLENTY Swivel Angle Mixers should be installed as soon as possible in all ARAMCO crude oil storage Tanks, to prevent sludge accumulation.

2) The starting point of the program should be JU'AYMAH and/or TERMINAL NORTH - RAS TANURA, being the most important storage areas. The consequences and implications of having to take one or more of these large Tanks out of service at any time are severe.

3) Mixers should be installed in the Tanks just before new Tanks are put into service, or just before existing Tanks are put back into service, following T. & I., or cleaning.

This is an ideal opportunity of installing the Mixers, without interruption to operations. The non-availability of an electrical supply should not delay the Mixer installation at this time, as to install the Mixers at a later date would be disruptive to operations, having to pump the Tank down, etc., whereas if the Mixers are installed while the Tank is already empty, the simple electrical connections can be made at anytime without interrupting operations.

4) Starting from the clean Tank condition, the first "charge" of oil will contain approximately 0.2% volume B.S. & W. If allowed to settle, the water could be drained off. Settling time will depend on operations and, therefore, the water draining will be a matter of "get as much as you can". The longer the Tank can settle, the more water likely to be drained.

The Mixer(s) should not be used until any water has been drained off, otherwise the water will be suspended in an emulsion and become difficult to remove.

The Mixers should then be started 8 to 10 hours before pump down and operated during pump down until the recommended minimum level for Mixer operation (approximately 10 feet) is reached. This method of operation will not necessitate the use of a demulsifier as the water has been drained off before the Mixers are started. Subsequent "charges" of oil should be treated in the same manner, but the Mixer angle should be changed every two to three months in accordance with the program laid down in the PLENTY Operating and Maintenance Instructions supplied with each Mixer, i.e. 30E left, 10E right, 20E left, 20E right, 10E left, 30E right, 30E left, etc., etc.

Crude oil Tanks which have been in service for periods up to 10 years without Mixers will certainly have sludge accumulated in the bottom. As proved during this mixer test this sludge can be suspended by Mixers, simply by following the procedure laid down during the test.

The one amendment recommended to the test procedure is that the demulsifier be added during the initial 48 hour mixing period, when the Mixer(s) are angled at 10E left of Tank center as this is the angle required for blending.

The mixer manufacturer should always be consulted on the size and number of Mixers required for a particular duty.

A second test should be carried out on a "Heavy" crude oil Tank to prove Mixers will suspend the more viscous sludge.

Mixers should be installed on a permanent basis as a long range test and evaluation on a few more Tanks. The 6 Tanks on Terminal North appear to be well suited, as they have water problems and they have suitable Mixer manways. However, some consideration should be given to a sewer or slops system around the crude oil Tanks if water draining is going to be a regular occurrence, as it is inevitable that some oil, albeit very small amounts, will be carried out with the water. Hence, the bund area near the drains would become dirty over a period of time.

TABLE OF CONTENTS

OBJECTIVE

The objective of this study is to determine the effectiveness of PLENTY swivel angle mixers in reducing volumes of accumulated sludge in crude oil tanks.

TANK DETAILS

No. 1141 Terminal South, Ras Tanura.

Pontoon type floating roof.

Conical bottom with slope 1 inch in 10 feet.

Mixer Manways: 24 ins. API 650 manways.

ORIGINAL TANK CONDITIONS

The tank was pumped down as far as possible removing all the oil leaving the sludge on the bottom.

Sludge Depths - measurements were taken independently.

 

A) 2.11.78 by N.A.O.E.

Method from the roof through each of the five emergency vents using a long pole with plywood disc on the end. The pole was lowered through the vent until slight resistance was felt, the pole was marked and then pushed through the sludge to the tank bottom where the measure was then taken. Average Sludge Depths : 24.5 ins.

B) 3.9.78 by Mixer Test Team

Method just prior to the mixers being installed with the mixer manways removed, a stick was pushed vertically through the sludge to the tank floor, removed and measured.

Sludge Depth : 22 ins.

With both mixer manways removed there was sufficient light to see that the sludge surface profile was basically flat.

SLUDGE SAMPLES : were taken through the open mixer manway using tin cans. Samples were collected from the top, middle and bottom of the sludge and sent to the laboratory for analysis.