Going back a number of years we used chevron packing 4 pieces with header and follower rings on hard chrome plated or ceramic plungers. worked well up to 15,000 psi. We never did get 20,000 psi working reliably but we used Aramid square section packing coiled like a spring.
We had for test a german pump that was rated to 65,000 psi that again used chevron packing but had a very slow plunger speed which may have been the key.
The above were all used on water.
What do you intend to pump ?
I expect the seal manufacturers are more genned up on these pressures than they were back then.
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