Storage Tank Level control 2

[Aus02]

I'm a young process engineer - I need to specify a low pressure (<100 mbarg) fixed roof (cone) storage tank. Tank is blanketed - blanketing used for pressure control. Capacity (from low to High level) is set by a process fluid residence time. Tank likely to be mounted on concrete plinth to supply NPSH for discharge pumps. Some water/crud expected to drop out in bottom but tank sizing not deterined by any dehydration requirement.

My questions (is there a rule of thumb or more formal guideline/standard):

1. At what level do I set bottom nozzles (to avoid the dead zone with water/crud)
2. At what levels should I set my Low low, low and high high level alarms?


Many thanks for your help on this

 

[saxon]

Aus02, Answers depend on the tank geometry, equipment layouts, process medium & storage characteristics, installed agitation equip., if any, heat transfer requirements, area for code requirements, etc.. None of which seems to be supplied. It gets real difficult to design anything when the basic information is missing.

Hope this helps.
saxon

 

[jxdcompaq]

Aus02, we have used the Davy process design manual as guideline in setting the liquid level control points for horizontal or vertical vessels. Keep in mind that these are guidelines and your engineering judgement should be foremost in the design.

 

[sshep]

If you have water/crud coming in it must have a way to get out. Some sort of a draw-off will have to be provided or nozzle elevation is irrelevant. You probably have similar installations on site to use a reference, but a low point sump with draw piping is common. This can go to sewer through a reasonable size drain valve- don't make too big because a very high drain flow is one of many ways to suck a tank in. The sump drain may also be piped up to your pump suction in addition to the normal suction nozzle in order to allow total pumpout of the tank. If you provide a water draw, your normal suction point can be placed low to maximize the useful volume of the tank.

I am not sure what a "plinth" is. Typically the level in the tank provides all the needed NPSHR, however, if you are elevating your tank, you open all sorts of design options (like cone bottoms) to deal with solids and cleaning problems that will have to be addressed after years of service.

Level settings, the instrumention to do them, shutdowns, etc depend on required reaction times, consequences, and control system infrastructure which are specific to your application. If you have continuous level coming into the panel, the normal high and low alarms can probably be associated with that. Any HH and LL alarms (or shutdowns) may need to be independent- no real recommendation is possible without more data.

 

[jproj]

In addition to saxon's comments...

1. At what level do I set bottom nozzles (to avoid the dead zone with water/crud)

Depends...
Does the tank have a flat bottom? What kind of "crud" are you talking about? Will the "crud" damage the pumps if it carries over into the suction piping? How often will the "crud" get cleaned out? What is the process fluid? Will the "crud" be suspended in the process fluid? etc...

All these factors will determine where your suction nozzle is located. If your "crud" will harm the pump (or downstream equipment), then you don't want to allow it into the suction piping, which means you will want to elevate the nozzle from the bottom of the tank. If the "crud" build up is high, and you can't remove if for long periods of time, your nozzle will need to be higher from the bottom of the tank. If the "crud" won't harm the pumps or downstream equipment then you can just put the nozzle flush with the bottom of the tank. On the other hand, if the "crud" is suspended in the process fluid, the location of the suction nozzle won't matter. I design tanks for boiler feedwater systems and the typical feed pump suction nozzle projection (above the bottom of the tank) is 3".

2. At what levels should I set my Low low, low and high high level alarms?

Again, depends...
Generally, the low alarms protect your pumps (keep them from cavitating), while high alarms protect your vessel (keeps it from overfilling / overpressurizing). So, it all depends on your system dynamics. If your tank drains very quickly when the pumps are running and there is no inlet flow, then you want to have some distance between your level alarms (to give the operators enough time to react). If, on the other hand, the level falls very slowly when the pumps are running and there is no inlet flow, your level alarms may only be a couple inches apart (4"-6"). Same goes for your high alarms (when there is inlet flow, but no outlet flow - most conservative design-wise). It also depends on your control system. If your alarms have pump starter / shut-off switches you will need less distance between alarms compared to a system that has manual pump controls.

My advice: Use common sense with safety being your #1 priority and consult with your manager or another engineer who has experience with your equipment.

Good luck!

jproj

 

[jay165]

Hey Aus:

I wanted to recognize you for posting an excellent question. You gave all the facts and units, listed your assumptions and why you're asking the question. Often the questions posted in this and other forums on this site are cryptic, nonsensical and don't have any further info to help a reader answer the question. In fact many indicate that original poster hasn't done any investigative work on their own and/or does'nt have a clue about what they're talking asking.

Good luck in your career. I'm sure you'll do fine and be an asset to our profession.

 

[MJCronin]

Aus02,

The design of the tank must be compatable with the maintenance program intended...

You should be aware that flat bottomed tanks are sometimes specified in a "coned bottom" configuration (typically, but not always "cone-up")

In the cone-up configuration, water, solids etc are forced to the shell of the tank where they are periodically drawn off.

Additionally, an older API standard (12C or 12F ??)had details for a "draw-off sump" that was also used for this purpose. This sump was a rectangular "bathtub" built into the tank floor to collect water/solids. An internal pipe led from the tank shell nozzle into the sump.

Also, contact PROTECTOSEAL, a company that makes a lot of tank accessories. They offer a "bump valve" (series 660)specifically for this purpose that may interest you.

http://www.protectoseal.com/specialty.html

Let us know what you decide.....

My opinion only....


MJC