Flow Meter

Without being able to modify the line during an outage, your options are quite limited, as you well know. Your best bet is to contact an engineering representative for one or more of the various instrument manufacturers.

You can search these out using globalspec. A couple of familiar companies:

Emerson Process Management's Rosemount
http://www.emersonprocess.com/rosemount/products/f low/index.html

Endress + Hauser
http://www.endress.com/

A useful beginner's introduction to flowmeters:
http://www.flowmeterdirectory.com/flowmeter_for_be ginner.html

Consider one of those paddle wheel turbine meters. They can be hot tapped without shutting down the system. Being a point measuring device, they will not be as accurate as an ultrasonic meter.

I assume you are using a rate of transit ultrasonic meter which measures the time it takes for an ultrasonic signal to travel upstream versus downstream. Consider a doppler ultrasonic meter. Essentially they measure the speed of particles in the fluid to calculate flow. They are not so good for clear flow, however entrained bubbles would serve as particles.

Since two months we are testing the new ultrosoncic flowmeter Nivusonic from the company Nivus (http://www.nivus.com)in parallel operation with a standard insertion transit-time-flowmeter. The sensors are mounted on the outside of a 1000mm pipe. The medium is wastewater. This system combines a transit-time method with a "Doppler"-method. If the concentration of bubbles becomes to high, the system can switch to velocity measurement of the particles with the "Doppler"-method. Up to date the results are very satisfying.

Spartek Systems out of Canada