US3762690A

US3762690A - Mixing system for floating roof tanks

Info

Publication number: US3762690A
Application number: US00221966A
Authority: US
Inventors: H Harrington
Original assignee: Mixing Equipment Co Inc
Current assignee: Mixing Equipment Co Inc
Priority date: 1972-01-31
Filing date: 1972-01-31
Publication date: 1973-10-02
Anticipated expiration: 1990-10-02
Also published as: DE2304225A1; NL7301363A; FR2170005B1; FR2170005A1; GB1397042A; DE2304225C2; NL176769B; JPS5611494B2; CA962999A; AU5150073A; NL176769C; AU469301B2; JPS4887462A

Abstract

Floating roof, liquid storage tanks such as for storage of petroleum products have a mixer motor in the center of the roof with a shaft extending through the roof, and a vertically pumping impeller arranged below the roof in the liquid to mix the liquid in the tank. This replaces and has many advantages over side entry mixers for similar tanks.

Description

United States Patent 1191 1111 3,762,690 Harrington 1 Oct. 2, 1973 [54] MIXING SYSTEM FOR FLOATING ROOF 3,411,756 11/1968 Ziegler 259/122 TANKS 2,931,534 4/1960 Wiggins. 220/26 R 3,329,301 7/1967 Lee 220/26 R [75] Inventor: H. Richard Harrington, Doylestown,

Pa. Primary ExaminerR0bert W. Jenkins [73] Asslgneez :lliilllg Equipment Co., Rochester, Attorney cumpston Shaw & Stephens [22] Filed: Jan. 31,1972

[21] Appl. No.: 221,966 [57] ABSTRACT Floating roof, liquid storage tanks such as for storage [52] US. Cl. 259/108, 220/26 R ofpetroleum products have a mixer motor in the center [51] IIIL Cl Bolf 7/22 of the roof with a Shaft extending through the roof7 and [58] Field of Search 259/121, 122, 124, a vertically pumping impeller arranged below the roof 259/] 1081 107; 220/26 R in the liquid to mix the liquid in the tank. This replaces and has many advantages over side entry mixers for [56] References Cited Similar tanks UNlTED STATES PATENTS 3,343,817 9/1967 Carangelo 259/122 10 Claims, 4 Drawing Figures PATENTED W 3.762.690

FIG- 2 FIG. 4

MIXING SYSTEM FOR FLOATING ROOF TANKS THE INVENTIVE IMPROVEMENT For many years it has been standard practice to use mixers in large tanks for storing petroleum products. The mixers keep sediment from settling and help keep the tank clean and serviceable. It has also been standard practice for many years that storage tank mixers were mounted on the side of the tank near the bottom and were flange-mounted through the tank wall. This created weaknesses and stress concentrations in the lower tank wall and threatened ruptures that would cause not only loss of the stored products, but also damage to the environment. Nevertheless, the prob lems of side entering mixers have been accepted and dealt with successfully for many years.

For at least 30 years, large liquid storage tanks such as for petroleum products have also used floating roofs that float on top of the stored product and rise and fall with its level inside a tank rim. This has hitherto not been associated with the mixing problem within the tank or the use of side-entry mixing units, except that such mixers interfere with maximum lowering of the roof, and increase the amount of unusable tank volume by a substantial amount.

The invention involves recognition of the possibility of mounting a tank mixer on the floating roof. At first impression, there were several serious problems involved in such an arrangement, and it was initially very doubtful that satisfactory mixing could be accomplished by a roof-mounted mixer. However, further study of the problem and laboratory experiments have shown that not only can satisfactory mixing be accomplished with a roof-mounted mixer, but safety requirements can be met, mounting and other problems can be solved, and the result has many surprising economic advantages over side-entry mixers.

SUMMARY OF THE INVENTION The inventive mixing system applies to a liquid storage tank having a floating roof, and it includes a mixer motor mounted on top of the roof over a through opening in the roof. A generally vertical'shaft turned by the motor extends through the roof opening into the liquid below, and an impeller is mounted on the shaft below the roofin the liquid for mixing the liquid. The impeller is configured to pump predominantly vertically to establish a vertical flow preferably in the center of the tank to induce radial flow along the top and bottom of the tank for full circulation. Flexible power connections lead to the motor, and a vapor lock is preferably formed between the roof and the shaft.

DRAWINGS FIG. I is a partially schematic, sectional elevational view of the interior ofa liquid storage tank using the inventive mixing system;

FIG. 2 is a partially schematic, partially cut-away elevational view of a vapor lock for the mixer shaft of FIG. 2; and

FIGS. 3 and 4 are partially schematic, fragmentary views of preferred liquid flow patterns for the inventive system.

DETAILED DESCRIPTION As shown in FIG. 1, storage tank is generally known and preferred for storing petroleum products. It

has a floating roof 1] that is also generally known for floating on the stored product and rising and falling with its level. Heretofore, liquid mixing within tank 10 was accomplished by side-entering mixers mounted in the lower wall of tank 10, but by the inventive system, a mixer 12 is mounted on roof 11 as illustrated.

Mixer 12 includes a mixer motor 13 mounted on a plate 14 preferably in the center of roof 11. Plate 14 covers a substantial central opening in roof 11, and as best shown in FIG. 2, has a small through opening 15 for receiving the drive shaft 16 of mixer 12. A preferably liquid-filled, vapor lock 17 surrounds shaft 16 for a vapor seal between shaft 16 and roof 11 at opening 15.

Shaft 16 extends below roof 11 into the liquid in tank 10 and supports an impeller 18 that is configured for vertical pumping. With mixer 12 in the center of roof 11 as preferred, vertical pumping by impeller 18 can keep the product in tank 10 well mixed and circulated. Plate 14 is preferably removable and large enough so that mixer 12 can be removed by lifting impeller 18 up through roof 11 through the opening normally coveredby plate 14. This can be accomplished with a lifting jib 19 such as shown in FIG. 1. Alternatively, plate 14 can be omitted, and mixer 12 with its shaft 16 and impeller 18 can be permanently installed in roof 11 to be removed or repaired only when tank 10 is drained and opened for inspection and repair.

One problem with side-entering mixers for tank 10 was that roof 1] could not be lowered below the level of the mixer units. This produced a relatively large heel or unusable volume in the bottom of tank 10, and this was a significant economic detriment. In circumstances requiring that the heel of tank Ill) be kept to a minimum, a sump or well 20 is preferably formed in the bottom center of tank 10 in registry with impeller 18, so that roof 11 can sink to its lowermost position without any interference from impeller 18. Another way to keep the heel of tank 10 to a minimum is to mount impeller 18 as close under roof 11 as feasible.

Motor 13 is powered through a power transmission conduit 21 leading in from outside tank 10. Motor 13 can be either electrical with conduit 21 carrying energized electrical conductors, or motor 13 can be hydraulic with conduit 21 carrying pipes filled with pressurized liquid. Either way, motor 13 rises and falls with roof 11 relative to tank 10 to require flexing of conduit 21 between tank 10 and motor 13. Several forms of flexible conduit are available, and there are several ways that can be arranged to carry power safelyto motor 13. Many safety requirements have to be met, and especially for an electric motor 13, electric or spark hazards must be avoided.

One preferred solution to the flexible conduit problem is shown in FIG. 1 where conduit 21 is fixed in place and rigid from motor 13 along the top of roof 11, along ladder 22, and along the outside of tank 10. Ladder 22 moves between top and bottom positions of roof 1] as illustrated and provides part of the conduit connection.

At the top of ladder 22 a flexible conduit connection includes a fixed post 23 having a swivel top 24 supporting a section of flexible conduit 25 connecting to rigid conduit 21 at the upper end of ladder 22. Flexible conduit 25 and swivel top 24 allow the illustrated conduit movement between the upper and lower positions of ladder 22 as illustrated.

At the bottom of ladder 22 on tank top 11, a rigid post 26 leads conduit 21 to a swivel top 27 from which a length of flexible conduit 28 leads to a support post 29 at the bottom of ladder 22. Flexible conduit section 28 moves between upper and lower positions of ladder 22 as illustrated, to keep a free loop of conduit between

posts

26 and 29 and safely out of contact with other equipment to allow rise and fall of roof Ill without hazard.

The circulation flow created in tank It) by impeller 18 is schematically represented in FIGS. 3 and 41 showing two alternative preferred embodiments. In FIG. 3, impeller 18 is oriented to pump upward to draw liquid up from the bottom of tank it) in a central column that is directed against the under side of roofllll and forced radially outward across the top of tank 10. This causes a downward flow around the walls of tank It) and an inwardly radial flow across the bottom of tank back to the uprising central flow to impeller 18. The result keeps the liquid in tank well mixed and prevents sediment settling. Such flow can be assisted by a draft tube and diffuser 30 arranged around impeller 18 as illustrated to help guide upflowing liquid into impeller 18 and to help speed the radially outward flow under roof 11. Draft tube and diffuser 30 can be omitted, and this is preferred, especially for minimum projection of mixing equipment down under roof 11.

The impeller 18 of FIG. 4 is oriented for downward pumping to establish a circulation pattern the reverse of FIG. 3. In FIG. 4, impeller 18 draws liquid radially inward under roof 11 and pumps liquid downward to the bottom center of tank 10 where the liquid turns radially outward to sweep across the bottom of tank 10 and rise along the outer walls to be drawn radially inward under roof 11 for another pass through impeller 18. The down-pumping impeller of FIG. 43 requires a little more clearance under roof l1 and is preferably spaced'below roof II by about its radius.

In addition to the advantages already mentioned, the inventive roof-mounted mixer system is superior to side-entry mixers in many ways. A well-designed mixer system in the center of a tank roof can mix the entire tank with one mixing unit, where the same size tank could require several side-entry mixers. The installation of a roof mixer is much simpler and safer than a side entry mixer which requires a very strong and well constructed flange mounting with a secure stuffing box forming a liquid seal around the mixer shaft. The roofmounted unit is also easier to maintain, partly because only a vapor lock is required around its shaft, and it can be removed without draining the tank. Side-entry mixers require a much higher minimum roof clearance above the bottom of tank 10 to make a much larger and more costly tank heel of unusable volume.

Higher safety standards for petroleum storage tanks also make roof-mounted mixer systems advantageous over side-entry mixers. In some places, a container wall is required around a petroleum storage tank, so that the wall will contain all the petroleum that flows from a rupture of a full tank. The container wall can sometimes be added most economically as a double-walled tank. This greatly increases the problems of mounting side-entry mixers, but does not affect roof-mounted mixer systems.

Once it has been established that a roof-mounted mixer system can accomplish the desired mixing, the advantages in such an assembly are apparent and signif icant. However, this solution was overlooked for many years in spite of the advantages.

Persons wishing to practice the invention should remember that other embodiments and variations can be adapted to particular circumstances. Even though one point of view is necessarily chosen in describing and defining the invention, this should not inhibit broader or related embodiments going beyond the semantic orientation of this application but falling within thespirit of the invention. For example, those skilled in the mixing art will appreciate the different mountings, conduit connections, impellers, draft tubes, and mixing devices usable in the inventive mixing system.

I claim:

1. An improved mixing system for a liquid storage tank having a floating roof, said system comprising:

a. a mixer motor mounted on top of said floating roof to travel up and down with said roof relative to said tank;

b. said roof having a through opening in the region of said motor;

c. a generally vertical shaft turned by said motor and extending through said roof opening into said liquid;

d. an impeller on said shaft a predetermined distance below said roofin said liquid for mixing said liquid;

e. said impeller being configured to pump predominantly vertically in said liquid;

f. means for supplying power to said motor from a remote source; and

g. said power supplying means including a power transmission line fixed to the outside of said tank, a power transmission line fixed relative to said floating roof, and a flexible power transmission line for connecting said fixed lines to accommodate relative motion between said fixed lines.

2. The system of claim I wherein said motor, said roof opening, and said shaft are approximately in the center of said roof.

3. The system of claim 2 including a vapor lock between said roof and said shaft in the region of said opening.

4. The system of claim I wherein said motor is electric and said power transmission lines comprise electric conduit.

5. The system of claim 1 wherein said motor is hydraulic and said power transmission lines comprise hydraulic conduit.

6. The system of claim 1 including a vapor lock between said roof and said shaft in the region of said opening.

7. The system of claim 1 including a well formed in the bottom of said tank below said impeller to receive' said impeller when said roof is in its lowermost position.

8. The system of claim 1 including a draft tube arranged under said roof around said impeller.

9. The system of claim 8 wherein said impeller is oriented to pump upward.

10. The system of claim 1 wherein said impeller is spaced below said roof by approximately the radius of said impeller.

Claims

1. An improved mixing system for a liquid storage tank having a floating roof, said system comprising: a. a mixer motor mounted on top of said floating roof to travel up and down with said roof relative to said tank; b. said roof having a through opening in the region of said motor; c. a generally vertical shaft turned by said motor and extending through said roof opening into said liquid; d. an impeller on said shAft a predetermined distance below said roof in said liquid for mixing said liquid; e. said impeller being configured to pump predominantly vertically in said liquid; f. means for supplying power to said motor from a remote source; and g. said power supplying means including a power transmission line fixed to the outside of said tank, a power transmission line fixed relative to said floating roof, and a flexible power transmission line for connecting said fixed lines to accommodate relative motion between said fixed lines.

2. The system of claim 1 wherein said motor, said roof opening, and said shaft are approximately in the center of said roof.

4. The system of claim 1 wherein said motor is electric and said power transmission lines comprise electric conduit.

7. The system of claim 1 including a well formed in the bottom of said tank below said impeller to receive said impeller when said roof is in its lowermost position.

9. The system of claim 8 wherein said impeller is oriented to pump upward.