If its full open then its almost zero. Perry has a value of 0.17 velocity heads for a gate valve (k) that equals that the equivalent length of pipe is 10 times the ID.
I would assume that a gate and a ball valve would have comparable loss coefficients at full open.
Ball valves can be tricky since they can come in either full bore or reduced bore design. You may want to determine the valve make and model and refer to the manufacturer's data for the actual valve Cv rating. For example....
Crane, as suggested by Art, is an excellent reference. If you don't have one, get it. You'll use it all your career if you stay in the technical side of engineering.
Full port ball valves can be estimated as an L/D of 3.
I take it that you mean 'ball' as in a spherical ball with a hole through it which sits within a body and they are generally quarter turn? The other type is a float operated valve like the sort you get in a water tank to fill it and it shuts off when the float has risen.
The term ball valve in the UK has changed in recent years and we separate the types by using the term 'float operated' (which I and half the industry still call a ball valve because we are old. Young engineers call the quarter turn valves 'ball valve'.
Float operated valves are tabulated in the Institute of Plumbing Guide (UK)or contact the manufacturer. Ball valves however are straight through usually and have minimal resistance. Just be careful where you use them because shutting or opening them quickly can result in some unhappy and expensive experiences..(floods, building damage, etc..recently a 12 storey accomodation block was flooded by an engineer turning a valve on too quickly)
