In Yajur Veda Mantra starting with "somah pitrumAn yamo angirasvAn......" the Pitrus referred to are the nitya Pitrus and not one's own pitru, pitAmaha and prapitAmaha. Also, in the nitya pitru tarpaNam, we use only water.
A person does tarpaNam using gingelly (til) for his own pitru, pitAmaha and prapitAmaha only when his father is not alive. In regard to nitya pitru tarpaNam forming part of brahma Yajna, it is not the case. Hence every vipra, irrespective of the fact whether his father is alive or not, has to perform nitya pitru tarpaNam as part of brahma Yajna if he belongs to Yajur Veda Apastamba SUtra. Thus performance of nitya pitru tarpaNam by such a vipra whose father is alive is in order.