Electronic – Q: ultrasonic sensors – distance detection

There are many types of ultrasonic flow flow meters such as in-line, clamp-on and Doppler flow-meters. As you cannot make any modification to your pipe, Clamp-on type is a good choice for your application. Ultrasonic flow metes have three main part which are sensors (ultrasonic transducers), electronic part (it includes the MCU and Time to digital converter) and the spool piece. In clamp-on type, first you must find suitable ultrasonic transducers that fit your requirements, and the rest is all about good measurement and processing of time difference between upstream and downstream signal from sensors (AMS, Maxim, TI and some Chinese company have special designed ICs for this purpose). You can mount sensors on your pipe, depend on the pipe size, one of the following configuration may be suitable for your application. To have an accurate flow meter, apart from designing process, you need to calibrate it and calibration systems are two much expensive.

You haven't really given any details on the sensors you're using so I'll just answer it in general.

An ultrasonic sensor does in fact send out a sound wave that spreads out. Basically, the sensor will register the time taken for the sound wave to bounce off the surface and return to the sensor. So only the nearest object will be detected. As a result of this, it's rather important to ensure that the wave emitted by it is perpendicular to the surface it strikes. Else, you'll get dodgy distance readings. The biggest upside of US sensors is that they get really good range. They also take awfully long though since sound travels really slowly. (~330 m/s)

IR sensors and laser sensors tend to be far more focussed. I don't imagine either of them is capable of sending out beams that only travel in straight lines but it is fair to treat them as such. So yes, these sensors will send waves in 'straight lines' to the reflecting substance and back. However, you can't really technically say straight lines because they often work based on the angle made between the reflected light and the detecting strip (smaller the nearer the sensor is to the reflecting surface). Measuring distance by time isn't really possible with these sensors because light is way too fast.