Need help in flow meter selection

[Asisraja D]

Hi professionals
We have planned to install flow meter for our RO water header lines but we are facing the flow range issue while selecting the flow meter.
Our pump capacity is 30 m3/hr (or) 30000 Liters per hour and pump head is 30 meters. we need to install this for 1 1/2" (40NB) line and 2 1/2" (65NB) line.
we have selected the metal tube rota meter for our application in this but the problem is for 40NB line they provide only 500 - 5000 LPH flow range and for 65NB line it is 1600 - 16000 LPH. why this much of variation in flow range for both sizes ? we need to measure the maximum flow rate of the pump as 30 m3/hr ?
is this possible in these two pipe sizes (40NB,65NB) ? As per vendors suggestion we could not achieve this much of flow rate in these sizes.
i will upload the vendors quotation here. can anyone please tell me why can't we achieve this flow rate ? As per Q=AXV when the pipe area reduces the velocity will increase so if we reduce the pipe area absolutely velocity will increase and same we calculated the head loss and it is very minimum range in 65NB pipe but huge head loss in 40NB line ? My doubt is due to huge head loss how can i calculate the flow rate in the 40NB line ? because then only i can select the flow range for flow meters. The pipe MOC is SS-304 ASTM grade A312.

i attached my pressure drop calculation here and vendor quotation please review when you have a free time.

Thank you all.

 

[Asisraja D]

For 40NB line head loss is very high as it is 9.68 bar but our head is 30 meters only.

if so 3.0 - 9.68 = -6.68 bar (negative pressure) i think i am getting wrong in this ?

 

[Asisraja D]

This is maximum flow range shared by vendor for (40NB,65NB) lines.

 

[mk3223]

You need to start the question with a sketch of the flow diagram from the source to the two flow meters with any proper instruments, including if any flow/pressure control valves in additional to the flow meters for better understanding of your system.

First, here are questions for you:
1. Typically the flow meter is to measure the flow range for a certain turn down ratio and better accuracy. Or, the larger size of the meter may be required for higher flow rate.

2. The flow velocities of two branches seem high in the design of the typical new facilitate. What are the required flow rates for 40NB and 65NB lines, instead of total 30m/h from the pump?

 

[LittleInch]

Asisraja,

I'm not actually sure where to start on this rather long and rambling post with no diagrams....

First, the nominal size of a pipe, especially at these sizes, is not the same as the actual ID, which is what you need in your calculations.
Rotameters do seem very popular in India for some reason, but for liquid flow they can be restricted as all the flow needs to go through them. They work on a set velocity, so min flow for your 40NB line in velocity terms (sq area ratio) is 2.6, so the 500min becomes 1500 and the 5000 becomes 16000. About the same velocity ratio.
So to measure 30m3/hr you prob need a 3" or 4" meter.

30m3/hr in a 40NB line for 100m is bonkers. You're doing 6.5m/sec. Pressure drop varies as a square of the velocity so no wonder it makes no sense.

Your question about negative pressure is so wrong I can't start to answer it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Asisraja D]

mk3223 and LittleInch sir
I attached my sketch with this and I searched about turn down ratio. I have included them with this.

Your question about negative pressure is so wrong I can't start to answer it.

So there will be a fixed or maximum flow rate possible in 40 NB line? Whatever the pump capacity is ( may be 50 m3/hr for example)
I could not achieve the maximum flow rate capacity of the pump in 40NB line.

I think I have to research and study more on this.

Thank you sir.

 

[georgeverghese]

There is only a single flow to be measured, as shown on your sketch. So why do you need 2 flow meters ? Think your sketch is incomplete, there is another branch stream out from the 65mm header ?

 

[mk3223]

Asisraja,
1. Determine the proper line size per the “required” flow rate on each section, which may not be the max pump flow.
2. Need to study the overall line system hydraulic and the proper design flow velocity/ pressure drop for sizing the lines of the header and each branch. The flow rate of each branch may be smaller than the pump discharge header.
3. Each branch sizing is based on how many users are on the branch line.
4. Decide how you to set up or control the flows of the header and branches, i.e. flow control or pressure control, and select the proper flow meters for better control and accuracy.
Hope these help.

 

[Asisraja D]

georgeverghese sir

there is another branch stream out from the 65mm header ?

We have taken a 40NB branch connection from 65NB header. That is where we are going to install a flow meter as I told two flow rates we have to know.How much flow rate in 65NB and 40NB ?

why do you need 2 flow meters ?
The 40NB header is for production block - 1.
65NB header is fir production block - 2.

So we have to assure and monitor the flow rare on daily basis.

 

[Asisraja D]

mk3223 sir

Determine the proper line size per the “required” flow rate on each section, which may not be the max pump flow.

As we didn't calculate the required flow rate for 40NB header unfortunately our manager selected pump capacity as 30m3/hr then we did headers.

 

[mk3223]

Asisraja,

It’s okay to have the pump of 30m3/h which may cover the necessary flow for all current users and the potential users in the future. Now the job is to size the header and branches to suit the operation needs.

For me, I may design the main discharge header size with the flow 30m3/h and the velocity 1.8m/s (about 6ft/s) to have the proper line size with pressure drop. I may not want to size the pipe with 4m/s or 6m/s velocities with higher pressure drops. To minimize the line pressure drop is to ensure the process pressure is enough at the end users with the possible future expansion.

For each branch, I need to know how many users and associated flows and calculate the branch pipe size accordingly with 1.8m/s flow rate.

Then, the size of the flow meters may be determined per line sizes accordingly, or per Vendor recommendation.

 

[pierreick]

Asisraja,
I'm sorry for you, your manager should manage and let the technical guys do their job.
A lot of good advice were given to you.
My 2 cents
Pierre

 

[LittleInch]

Asiraja,

I appreciate the position you are in, but please be aware that this is apparently being one ALL WRONG.

You do not randomly pluck out of thin air flow rates, pump head, line sizes or metering types which is what seems to be happening and then try to make the system work.

You or someone else before you creates a design basis and list things like:
Design code
Pipe sizes and ID options
Design flow (maximumm flow from the RO unit or demand)
Inlet head or pressure (range) into a pump
Location / length of users from the pump outlet
Pressure required or head difference from pump to the end user
Flow to different users if you have more than one.

THEN you design your system to meet the actual requirements balancing cost vs performance. In most cases this work out to be a flow velocity between 1.5 to 2.5 m/sec for your type of flow.

If some of this information is not available then you need to make some sensible assumptions or indeed back calculate so that e.g. for a 40NB line at different flows needs different pressures / head. Now your manager has apparently chosen the pump then this gives you at least one known parameter (flow and head at one point on the pump curve). Now you need to fill in the rest such as a maximum flow at the same time to blocks 1 and 2, a pump curve etc.

Meter type is many and various. Rotameters are only one and have some faults and limitations, but maybe what is commonly used. But there are orifice meters, mag flow, ultrasonic, turbine meters etc etc.

Without good data you cannot design anything useful.
I would start again from scratch if I was you....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Asisraja D]

pierreick sir
I don't know why they are doing like this without any proper design. They did header implementation and then without proper knowledge they go for pump selection and rest of the works.

Finally we will have to manage them.

 

[Asisraja D]

LittleInch sir

Without good data you cannot design anything useful.

This is the part that I will have to focus more because as a project engineer I have to implement my day to day works without any delay if so this is another part of my job to check and verify our manager's improper design in piping. I am learning something daily through this forums. Because of you guys I have learned a lot.

Thank you for your continuous support.

 

[pierreick]

Hi,
You can use this calculator to perform hydraulic calculations.
To me, you should start from scratch, with the right velocity to size the pipe.

https://checalc.com/calc/pump.html

Pierre