8 replies to this topic

[sunny143]

Dear,

 

I am dealing with the level setting of Internal Floating roof tank. Currently i am applying following criteria for tank outlet nozzle location.

1. Pump suction nozzle height is minimum 300mm from bottom plate of tank.

2. LLLL (Low Low Liquid Level) = 1.5 D from central line of pump suction nozzle or 100mm from top of suction nozzle (whichever is greater)

For Example: From tank Bottom = 300mm + 2 x Suction NOzzle ID

.

My question is, what if i adjust this height to minimum level like 150mm + Suction Nozzle ID? I know it will cause low NPSH(a) but i want to know any practical Hazard/contigency concerned with this thing as per your experience. 

 

Guys your's prompt response will be highly appreciated.

 

[fallah]

sunny,

 

The common concerns could be as follows:

 

1- Lower NPSHA

2- Sucking the bottom sluge

3- More severe vortex

4- Lowering the pump's level of installation

[sunny143]

@Fallah:

Dear i knew these consequences, but i wanted to know that shall i consider this lowered criteria as a design engineer due to small size of the tank? 

[srvna90]

Dear Sunny, 

 

The whole concept of having a LLLL alarm in a system is

1. To avoid the above said causes and to save the pump from cavitation problem.

2. There will be accumulation of sludge and other impurities which may affect the downstream process.

3. The inventory left out in such a Level would be mostly one or two hour consumption of the Process so wont imply more financial effect also.

4. In few tanks that come with servo level indication, the weight of the servo level indication is positioned at a height and pumping the liquid lesser than the height may result in cutting of the wire which connects the Weight of the servo level.

[fallah]

@Fallah:

Dear i knew these consequences, but i wanted to know that shall i consider this lowered criteria as a design engineer due to small size of the tank? 

 

sunny,

 

It could be investigated when whole system info, such as tank dimentions, handling fluid, pump data sheet, piping configuration...., would be in hand...

[srvna90]

Dear Mr. Fallah, 

 

Decreasing the LLLL wont give any significance to the over all operation as Mentioned by Sunny that its a small tank. When the tank is small the left over volume is going to meager which will not of no use to the process. 

 

Its a situation where no advantage or gain can be expected by changing the design.

[sunny143]

Lower liquid level could potentially increase the risk of exposing the suction nozzle to vapour space and sucking vapour into the pump. But this risk could be ignored due to the following:

 

1- Tank is atmospheric

2- As condensate is stabilized so no chances of vapor generation within in the system.

3- Blanketing is not available

4- Pump tripping is carried out by low suction pressure

5- Tank level transmiters are SIL-3 rated.

[sunny143]

 

@Fallah:

Dear i knew these consequences, but i wanted to know that shall i consider this lowered criteria as a design engineer due to small size of the tank? 

 

sunny,

 

It could be investigated when whole system info, such as tank dimentions, handling fluid, pump data sheet, piping configuration...., would be in hand...

 

Lower liquid level could potentially increase the risk of exposing the suction nozzle to vapour space and sucking vapour into the pump. But this risk could be ignored due to the following:

 

1- Tank is atmospheric

2- As condensate is stabilized so no chances of vapor generation within in the system.

3- Blanketing is not available

4- Pump tripping is carried out by low suction pressure

5- Tank level transmiters are SIL-3 rated.

[fallah]

  

Lower liquid level could potentially increase the risk of exposing the suction nozzle to vapour space and sucking vapour into the pump. But this risk could be ignored due to the following:

 

1- Tank is atmospheric

2- As condensate is stabilized so no chances of vapor generation within in the system.

3- Blanketing is not available

4- Pump tripping is carried out by low suction pressure

5- Tank level transmiters are SIL-3 rated.

 

 

sunny,

 

What about the reduction in pump's NPSHA?