Tank Blending

Figure 1 - Blending can be improved by a shrouded blending nozzle system because the jet nozzle educts a large quantity of surrounding fluid.

This document discusses a simple low cost tank blending method. It is referred to as a shrouded blending nozzle system, and it works quite well. The shroud causes the jet nozzle to educt a large quantity of surrounding fluid, improving blending. A simplified diagram of the nozzle portion is shown in figure 1 above.

Make the nozzle point upward at an angle such that a straight line projected from the nozzle would hit the liquid surface 1/2 to 2/3 of the way across the tank diameter. The idea is to promote top to bottom turnover. However, tilt the nozzle slightly to the left to promote a slight swirl effect. Aim the nozzle at a point about 1/3 of a radius off-center.

The following rules of thumb apply primarily to the situation where a final tank requires blending after, and perhaps during, transfer from rundown tanks.

1. Base tank size 10,000 barrels.
2. Use about 25 hp circulating pump.
3. Provide roughly 50-75 feet of head or a little higher.
4. For size other than 10,000 barrels, ratio directly for horsepower.
5. Circulation rate in GPM can be calculated from horsepower and head.

Size jet nozzle outlet diameter with Δh = u2/2g

Where:
Δh = Head loss in feet of flowing fluid
u = Velocity in ft/sec
g = 32.2 ft/s2

Often in refrigerated storage light hydrocarbons are involved which have a large gravity change with temperature. This condition makes such storage relatively easy to stratify. Be sure not to put in too large a blending pump for refrigerated storage. Not only does it waste blending pump horsepower, but extra heat is added to the fluid that must be removed by the refrigeration system (a double penalty). Operators often tend to leave blending systems running full time.

Provide a means of filling the tank to operating level prior to operating the blending nozzle system. Damage could result from discharging the high velocity jet into an empty tank;

When circulating the blending system and running down into the tank at the same time, it may be possible to direct the rundown stream into the circulating pump suction for additional blending in the pump.