@SLH: section 5.5.2. of API 610-10th edition (I use the ISO version ISO 13709 btw) says for inline pumps forces should be less than 2x table 4 values. That's also the check CII does.
If this isn't satisfactory Appendix F, part F.2. gives extra rules for the forces.
My nozzle forces are more than 2x table 4 values, so I have to use app. F.2 to check them.
I think I know enough atm. I've modelled the pumps as inline pumps with a flexible support (piece of small pipe), and checked the nozzle forces according to app. F.2 which makes them pass.
I'll give the forces to the manufacturer for a double-check.

Thanks all for your replies.